LSP: Support textDocument/SemanticTokens/range request

SemanticTokens/range allows us to request tokens for a given range. For
us, this means "Current View Lines". This greatly reduces the load on
both us and the server as now we aren't processing tokens for full
document which can easily mean thousand of ranges per typed character.
Unfortunately, clangd currently doesn't support this however
rust-analyzer and dart do support it.