Use proper JSONC grammar

[DecimalTurn]

Description

At the moment, the Javascript grammar is used for JSONC (JSON with comments). This is not ideal since it makes the keys have the same highlighting as strings making it less pleasant to read:

Looking at the official grammar used by VS Code, we get a nicer experience:

This PR changes the grammar to use the official VS Code grammar for JSONC. However, to avoid adding the whole VS Code repo as a submodule, I've used a "mirror" repo that remains synced with the official version.

Checklist:

• I am changing the source of a syntax highlighting grammar
  • Old: https://github.com/atom/language-javascript
  • New: https://github.com/DecimalTurn/vscode-jsonc-syntax-highlighting

[lildude]

LGTM. Thanks.

Important

The changes in this PR will not appear on GitHub until the next release has been made and deployed. See here for more details.