x/tools/gopls: add go:embed support

[hamidfzm]

Is your feature request related to a problem? Please describe.
Go embed feature introduced in version 1.16. Currently, go-vscode treats text like //go:embed app as a simple comment.

Describe the solution you'd like
Provide text highlighting or navigation by CTRL+Clicking on these comments

Describe alternatives you've considered
Goland provides this feature in its recent versions:

Additional context
VS code view

It would be really nice if we have this feature in my favorite code editor.

[findleyr]

This could be implemented with a code action+associated command that invokes window/showDocument. It could also be implemented with jump-to-definition+hover. I am not sure which is best, but this seems to belong in the gopls issue tracker.

[hyangah]

For individual files, code action associated with window/showDocument or textDocument/documentLink would work. We still need a solution for handling folder type resources. For VSCode, I filed an issue microsoft/vscode#141564.

[ansaba]

List of improvements

• checks for the "embed" import
• Report it as semantic for highlighting
• The directive must immediately precede a line containing the declaration of a single variable. Only blank lines and ‘//’ line comments are permitted between the directive and the declaration.

EDIT (rfindley): cross out semantic highlighting, as I think we should punt on that for now

[findleyr]

We should also include a quick-fix to add the missing "embed" import.