cmd/govim: use gopls registered file globs for watching

[myitcv]

gopls calls the client via RegisterCapability to indicate what file globs should be watched. e.g.

RegisterCapability: &protocol.RegistrationParams{
    Registrations: {
        {
            ID:              "workspace/didChangeWatchedFiles-0",
            Method:          "workspace/didChangeWatchedFiles",
            RegisterOptions: map[string]interface {}{
                "watchers": []interface {}{
                    map[string]interface {}{
                        "globPattern": "**/*.{go,mod,sum}",
                        "kind":        float64(7),
                    },
                    map[string]interface {}{
                        "globPattern": "/home/myitcv/gostuff/src/github.com/myitcv/playground/**/*.{go,mod,sum}",
                        "kind":        float64(7),
                    },
                    map[string]interface {}{
                        "globPattern": "/home/myitcv/gostuff/src/github.com/myitcv/playground/blah/**/*.{go,mod,sum}",
                        "kind":        float64(7),
                    },
                },
            },
        },
    },
}

It then calls UnregisterCapability (with a corresponding ID) when the client should drop a watcher:

UnregisterCapability: &protocol.UnregistrationParams{
    Unregisterations: {
        {ID:"workspace/didChangeWatchedFiles-0", Method:"workspace/didChangeWatchedFiles"},
    },
}

Currently our file watcher is rather dumb and just tries to watch everything (ignoring submodules) under the workspace root. However, per #604, even this is not complete.

So closing #604 in favour of this and noting the following enhancements:

• new directories should be recursively added to the watcher subject to them matching the glob supplied by gopls
• if the glob sent by gopls can't be made more specific (to ignore sub modules for example, or . prefixed directories, etc) then we could implement this client-side