Make filename relative

[benmccann]

My main motivation for this PR is that sveltejs/svelte#4856 would embed the filename in the compiled code. I don't think that should be an absolute path because that makes it so that builds are not reproducible (e.g. compiled file will be different if you build in different directory or different machine), can leak information (e.g. my username is in my path), etc.

The filename is used in three existing places today:

• debug - I think relative path is easier to read for debugging and drops the redundant information
• sourcemap - the examples I see online use relative paths (e.g. bugsnag article/tutorial), so it seems like an improvement there
• passed to preprocess - I did an npm link of this PR against svelte-preprocess and all tests passed.

[AlbertMarashi]

Looks good 👍

[lukeed]

Does this require sveltejs/svelte#5476 parity?

[benmccann]

No. Other way around. I didn't want to merge that PR until we had this change

[lukeed]

I checked this out locally & applied it to a project using PostCSS and TypeScript preprocessing. Everything seems to check out well.

Could not validate the claims made by the link svelte PR, but only because my test app is considerably smaller.