Implement syntax highlighting

[certik]

There are 3 different ways to do this:

• in the typescript vscode extension itself
• in the LSP by writing a dedicated parser just for syntax highlighting
• in the LSP using our existing parser

We'll start by using our existing parser, since it is the most versatile way: no code duplication, and because the LSP is so fast, this should be fast enough that the first approach is not needed, so we'll keep things simple. The first approach is typically used when the LSP is slow, so it gives you a fast approximate syntax highlighting, then accurate later. I think we can give the exact highlighting right away.