bug(cli): #1323 refine serving logic for node modules plugin

[thescientist13]

Related Issue

#1323

As part of exploring Bun support - thescientist13/greenwood-bun#5 (comment)

Documentation

N / A

Summary of Changes

1. Make sure Node Modules resource plugin is only serving files from node_modules
2. Misc plugin tweaks
   • standardize HTML resource plugin extension
   • check for Content-Type header first in Active Content resource plugin

TODO

1. any chance this addresses restore custom loader and import attribute test case for Lit renderer plugin #1463 ?
2. somewhat related, technically tsconfig.json supports JSON5, which means things like comments and dangling commas will break here (e.g. for Greenwood, a tsconfig.json will have to be JSON serializable)