feat: use builtin neovim semantic token api

[stefanboca]

Closes #187

[stefanboca]

I'm not sure how to test this, so I've marked it as draft...

Some notes:

• I made the assumption that there is only ever a single semantic bracket request at a given time. Any new requests (calls to add_brackets_via_semantic_tokens that occur before a previous timeout expires will be overwritten.
• semantic tokens must not be disabled by users - the way to do this was by setting client.server_capabilities.semanticTokensProvider = nil, which we have a nil check for, so this should not change any behavior.

[stefanboca]

Previously this line was cursor[2] <= token-end_col, but vim.lsp.semantic_tokens.get_at_pos does the equivalent of the current version. This might have been an off by one error originally? Maybe it's also related to #1168?