Merge #1900 #1907

1900: Add rollup bundler for vscode extension r=matklad a=JasperDeSutter

This is an alternative approach to #1451 - bundling should improve extension startup times as explained [here](https://code.visualstudio.com/api/working-with-extensions/bundling-extension).

I'm using [rollup.js](https://rollupjs.org/guide/en/) which is a small and light bundler as opposed to webpack. Bundling is only done for creating the final vsix package, reducing 196 files down to 7 (of which 1 javascript file).

1907: update documentation according to new settings r=matklad a=lesurp

1. `command` has been added to change the default cargo-watch command
2. `check-arguments` has been renamed to `arguments` as a consequence

Thoses changes were merged in #1434

Co-authored-by: JasperDeSutter <[email protected]>
Co-authored-by: Paul Lesur <[email protected]>