[Webpack5] Using old watchOptions regexp & removing Node 13 support

[weaverryan]

2 quick, minor follow-ups from #645.

• Removed Node 13, as it is no longer supported
• Changed the ignored back to a regular expression, which it was originally and once again a regexp works: Update Webpack to v5 (+ other dependencies) #645 (comment)
• Upgraded to Stimulus 2, which the stimulus bridge now depends on