Fix semicolon on package (#184)

Problem
-------
Package declaration with semicolon fails to parse.

Solution
--------
This adds an optional semicolon to fix that.

corpus/definitions.txt

@@ -126,6 +126,17 @@ package c {
     (template_body
       (object_definition (identifier)))))
 
+================================
+Package with comma
+================================
+
+package a.b;
+
+---
+
+(compilation_unit
+  (package_clause (package_identifier (identifier) (identifier))))
+
 ================================
 Package (Scala 3 syntax)
 ================================

grammar.js

@@ -164,6 +164,7 @@ module.exports = grammar({
     package_clause: $ => seq(
       'package',
       field('name', $.package_identifier),
+      optional($._semicolon),
       // This is slightly more permissive than the EBNF in that it allows any
       // kind of delcaration inside of the package blocks. As we're more
       // concerned with the structure rather than the validity of the program