Omit non-simple-identifier names in scopeCompletionSource

FIX: Make sure `scopeCompletionSource` doesn't try to complete property names
that aren't simple identifier (such as numeric indices).

See https://discuss.codemirror.net/t/javascripts-scopecompletionsource-yields-properties-that-are-inaccessable-with-the-obj-prop-syntax/6606

Author: marijnh

src/complete.ts

@@ -136,7 +136,7 @@ function enumeratePropertyCompletions(obj: any, top: boolean): readonly Completi
   let options = [], seen: Set<string> = new Set
   for (let depth = 0;; depth++) {
     for (let name of (Object.getOwnPropertyNames || Object.keys)(obj)) {
-      if (seen.has(name)) continue
+      if (!/^[a-zA-Z_$][\w$]*$/.test(name) || seen.has(name)) continue
       seen.add(name)
       let value
       try { value = obj[name] }