x/tools/gopls: don't register semantic tokens capability unless enabled

[stamblerre]

We can register/unregister it dynamically, since it is gated by a flag. We just don't want the client to send semantic tokens requests if the user has disabled semantic tokens.

/cc @pjweinb

[stamblerre]

I think that before we fix this, we need to resolve #41966 by removing the per-view configuration. Semantic tokens are a per-session option, which is why the dynamic configuration is currently tricky to implement.

[gopherbot]

Change https://golang.org/cl/263525 mentions this issue: internal/lsp: dynamically register semantic tokens