gopls/fake: add semantic token modifiers to fake editor

This change will make it possible to do semantic token regtests.

Change-Id: I9963c60f61af30f973a2ee4cd32aaa5545bdc4ec
Reviewed-on: https://go-review.googlesource.com/c/tools/+/448296
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Peter Weinberger <pjw@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>

gopls/internal/lsp/fake/editor.go

@@ -265,6 +265,10 @@ func (e *Editor) initialize(ctx context.Context) error {
 		"event", "function", "method", "macro", "keyword", "modifier", "comment",
 		"string", "number", "regexp", "operator",
 	}
+	params.Capabilities.TextDocument.SemanticTokens.TokenModifiers = []string{
+		"declaration", "definition", "readonly", "static",
+		"deprecated", "abstract", "async", "modification", "documentation", "defaultLibrary",
+	}
 
 	// This is a bit of a hack, since the fake editor doesn't actually support
 	// watching changed files that match a specific glob pattern. However, the