Add highlights.scm

[ikatyang]

Context: #19 (comment)

[kkharji]

🤩🤩

[b3nj5m1n]

Afaik, injections.scm is also universal to all tree-sitter implementations, and since Markdown's fenced code blocks are quite important I think it would make sense to also include this file in this repository.

There is already one in the nvim-treesitter plugin, see here, however this is specific to neovim. See this issue for a little more context.

[ikatyang]

Ref: https://tree-sitter.github.io/tree-sitter/syntax-highlighting

There are 3 officially supported queries: highlights.scm, locals.scm, injections.scm.