Add $ as a trigger character

This makes string interpolation completions much more discoverable and I
haven't yet found any bad outcome for this.

At least in VS code

  * With default settings, `s"Hello $|"` with cursor at `|` triggers a
    completion

  * With (non-default) `"editor.quickSuggestions.strings": true`,
    `"Hello $|"` with cursor at `|` triggers a completion (and thanks to
    the interpolation logic, that completion will add in the `s` string
    interpolation context)

metals/src/main/scala/scala/meta/internal/metals/WorkspaceLspService.scala

@@ -1180,7 +1180,7 @@ class WorkspaceLspService(
         capabilities.setCompletionProvider(
           new lsp4j.CompletionOptions(
             clientConfig.isCompletionItemResolve(),
-            List(".", "*").asJava,
+            List(".", "*", "$").asJava,
           )
         )
         capabilities.setCallHierarchyProvider(true)