x/tools/gopls: use progress notifications for the initial workspace load

[axetroy]

Is your feature request related to a problem? Please describe.

vscode-go initialization takes a while. During this time, there is no grammar prompt and nothing is ready.

I don’t know when it will be ready, it may depend on the size of the project

If it shows a loading state, this can alleviate user anxiety. At least let me know that it is working

Describe the solution you'd like

like Typescript extension does

Describe alternatives you've considered

None

Additional context

None

[gopherbot]

Change https://golang.org/cl/248623 mentions this issue: internal/lsp: adds initial workspace load notification