Mark Parser constructor as protected so plugins can extend it

FIX: Mark the `Parser` constructor as protected, not private,
so plugins can extend it without type errors.

Closes #1282

acorn/src/acorn.d.ts

@@ -733,7 +733,7 @@ export class Parser {
   options: Options
   input: string
 
-  private constructor(options: Options, input: string, startPos?: number)
+  protected constructor(options: Options, input: string, startPos?: number)
   parse(): Program
 
   static parse(input: string, options: Options): Program