feat: pass current CLI public lib path to webpack env

[rigor789]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

There is currently no way to get values from the nativescript.config.ts file in the webpack config unless you use a .js config file.

What is the new behavior?

With this change, the path to the nativescript lib (exposing the public API of the CLI) is passed to the webpack env, and the webpack config can read useful data from the CLI - inlcluding values from a ts config file. For example, the following would get the values from the nativescript.config.ts (or .js).

const nativescriptLib = require(env.nativescriptLibPath)
const nsConfig = nativescriptLib.projectDataService.getProjectData().nsConfig
console.log(nsConfig)

Or even using the config service directly:

const nativescriptLib = require(env.nativescriptLibPath)
console.log(nativescriptLib.projectConfigService.readConfig())
console.log(nativescriptLib.projectConfigService.getValue('id'))
// setValue is also exported for updating the config!

/cc @farfromrefug