Add TextMate grammars, language server & VS Code extension

[eemeli]

WIP, will probably get modified and rebased. Various parts of the extension are still pretty buggy.

To play around with this, check out the mf2-vscode branch, and run at the repo root:

npm install
npm run build:vscode

Then open the repo root in VS Code and press F5 to "Start Debugging". This should open a new window where the extension is active. You can then find mf2 and mf2-message as language options. You may also use this demo file for a less empty starting point.