fix: parse options ending with 3+ hyphens

Before this commit, options ending with three or more hyphens were
being parsed as positional arguments, because a regular expression
didn't have a start anchor.

Closes yargs#433

lib/yargs-parser.ts

@@ -225,7 +225,7 @@ export class YargsParser {
       if (arg !== '--' && isUnknownOptionAsArg(arg)) {
         pushPositional(arg)
       // ---, ---=, ----, etc,
-      } else if (truncatedArg.match(/---+(=|$)/)) {
+      } else if (truncatedArg.match(/^---+(=|$)/)) {
         // options without key name are invalid.
         pushPositional(arg)
         continue