cmd/govim: add support for runtime configuration of WorkSpace folders

[myitcv]

Currently, users are required to open Vim with a working directory that is within a module context (or GOPATH in GOPATH-mode) to be able to do anything.

This, however, is overly and unnecessarily restrictive. Some users use plugins that allow them to operate Vim more like VSCode. i.e. open once (with a working directory of, say, $HOME), and then open projects from there via file explorers and the like. Those projects will likely be modules (or subdirectories of GOPATH).

For this to work however, we need to support runtime configuration of WorkSpace folders from Vim via govim. WorkSpace folders tell gopls where things are rooted (typically modules). "Opening a workspace folder" represents establishing a new root. gopls then attempts to "do the right thing" by routing file operations to the most appropriate view (gopls concept that enables multiple WorkSpaces folders).

Depends on golang/go#31635

[myitcv]

This landed in https://go-review.googlesource.com/c/tools/+/175477/

[myitcv]

See also golang/go#33347 (comment)

[myitcv]

Related to #428

[myitcv]

Per conversation with @idawes on Slack, this also needs to encompass some form of persistence for these preferences/setup. Suggested formats were a CUE config file that contained definitions for the workspaces beneath it (along with details like build tag preferences etc).

[myitcv]

We should also only trigger this when the server registers an interest via the RegisterCapability call to the client.