dev(ranger): add action for extension-request

[jsjoeio]

This PR helps automate how we handle extension request issues. It also updates the docs for installing OpenVSX extensions via the command line.

TODOS

• refactor docs based on feedback

[code-asher]

I think this overlaps a little with the previous paragraph that lists how to install extensions on the CLI.

What if we move that into this heading, then create another heading for this content with something like "Installing OpenVSX extensions via the command line" for this content?

We could also mention they can switch early to OpenVSX by setting SERVICE_URL and ITEM_URL (we already have a section generally about the marketplace url so maybe we just link it from here).

[code-asher]

Actually maybe we should put this under "How can I request a missing extension?"

[jsjoeio]

Agreed.

Yes! Let's do that!

[code-asher]

Or just remove the extension request section.

[jsjoeio]

I opted to add it under "How can I request a missing extension" - I say we move remove the extension request section once we fully switch to OpenVSX

[code-asher]

I think we should tweak it a little more because we have some overlap in the instructions and we recommend opening an issue which we will just close.

Also, what do you think about recommending using the environment variables to download from Open VSX rather than the curl command? That way they could do:

SERVICE_URL=bla ITEM_URL=bla code-server --install-extension <id>

I'm not completely sure if that's easier or harder to use though. 🤔 If you set the variables once then it's easier in the long term but I think it might be a bit more typing in the short term.

Here's a really rough draft for the general structure that's knocking about in my head:

## Installing an extension 

Extensions can be installed from the marketplace using the extensions sidebar in
code-server or from the command line:

```
code-server --install-extension <extension id>
```

## How can I request a missing extension?

We are currently in the process of transitioning to [Open VSX](https://open-vsx.org/). 
Once https://github.com/eclipse/openvsx/issues/249
is implemented, we can fully make this transition. Therefore, we are no longer
accepting new requests for extension requests.

Instead, we suggest one of the following:
  - [Switch to Open VSX](#how-do-i-configure-the-marketplace-url) now 
  - Download and [install the extension manually](#installing-an-extension-manually)

## Installing an extension manually

If an extension is not available from the marketplace or does not work, you can
grab its VSIX from its GitHub releases or build it yourself.

Once you have downloaded the VSIX to the remote machine you can either:
  - Run the `Extensions: Install from VSIX` command in the Command Palette.
  - Use `code-server --install-extension <path to vsix>`

## How do I configure the marketplace URL?

It is possible to point code-server to another extension marketplace it by
setting the`SERVICE_URL` and `ITEM_URL` environment variables. These correspond
directly to `serviceUrl` and `itemUrl` in VS Code's `product.json`.

e.g. to use [open-vsx.org](https://open-vsx.org):

```bash
export SERVICE_URL=https://open-vsx.org/vscode/gallery
export ITEM_URL=https://open-vsx.org/vscode/item
```

These environment variables will also work with the `--install-extension` flag.

While you can technically use Microsoft's marketplace with these, please do not
do so as it is against their terms of use. See
[above](#differences-compared-to-vs-code) and this discussion regarding the use
of the Microsoft URLs in forks:

https://github.com/microsoft/vscode/issues/31168#issue-244533026

See also [VSCodium's docs](https://github.com/VSCodium/vscodium/blob/master/DOCS.md#extensions--marketplace).

## Where are extensions stored?

Defaults to `~/.local/share/code-server/extensions`.

If the `XDG_DATA_HOME` environment variable is set the data directory will be
`$XDG_DATA_HOME/code-server/extensions`. In general we try to follow the XDG
directory spec.

## How is this different from VS Code Codespaces?

[code-asher]

My thinking is that the flow from a user might look like:

"How do I install an extension?"
"Oh, it's not available, how do I request it?"
"Ah, they aren't accepting requests, how do I install it then?"
"OK, I can either do it manually or download from Open VSX."

[jsjoeio]

Ahh yeah good point about that. Okay I'll make these changes then re-request a review! Thanks for adding the structure/flow too! That helps a ton!!

[jsjoeio]

Embarrassingly, I didn't know you could do it this way, hence the curl suggestion:

SERVICE_URL=https://open-vsx.org/vscode/gallery ITEM_URL=https://open-vsx.org/vscode/item code-server --install-extension stylelint.vscode-stylelint

[jsjoeio]

bump @code-asher ready for another review