Implement textDocument/semanticTokens/full

[martskins]

This PR implements the textDocument/semanticTokens/full LSP method and removes the old "deprecated" (didn't ever really land as an accepted feature of LSP) implementation.

This also sets a default generic mapping of LSP token types to VIM highlight groups that will be merged with the user-provided language-id-specific semantic tokens mappings config which is used to decide which highlight group to assign to each token.

This is still a draft and there a few things left for this to be considered "working":

• [x] Token modifiers
• [ ] ~Map all token types to a vim highlight group~
• [x] Call the function to get the semantic tokens on file save/change/open/etc
• [x] Decide on whether we want to map to vim highlight groups or create our own LCN specific highlight groups that we can link to VIM's generic ones.
• [x] Update user documentation to reflect the removal of textDocument/semanticHighlight and the addition of this one. Also might be a good idea to add a link to the LSP documentation listing all the different token types if we end up supporting all of them.
• [ ] ~Support for textDocument/semanticTokens/range (this I believe we could leave out of the first release of this feature)~
• [ ] ~Support for textDocument/semanticTokens/full/delta (this could also be optional for the first release)~

Closes #1193

[martskins]

I think the way to go here is use standard vim highlight groups, not LCN defined ones. If users want to define their own groups that's fine, but we have no reason to over engineer this I think. I'm gonna assume we take that path unless someone has a compelling reason not to do so.

[martskins]

For the sake of releasing this faster I'm gonna leave the default mappings of LSP tokens to vim highlight groups for a later PR. I'm a little undecided on what the proper mappings are so I guess it could be useful to get some more eyes on that front.