x/tools/gopls: code lens for interfaces implemented by methods

[guettli]

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

If I am looking at existing Go code, I would like to see immediately if this method was
implemented to satisify an interface.

For example like this screenshot from a different IDE:

the red up-arrow symbol on the left side shows that this method implements an interface.

It would be great to have these indicators next to the line number in vscode.

Describe alternatives you've considered

I thought about alternatives, but having a small indictor next to the line number is the only solution I have found.

Related discusssion: golang/vscode-go#1735

[findleyr]

CC @adonovan, who has just completely reimplemented our implementations feature...

This is similar to the oft-requested feature to show the number of references via a code-lens. Both would have been impossible prior to indexing, but may be possible now.

Transferring to the gopls issue tracker.

[findleyr]

@hyangah points out that this is a dupe of #56695.