build: support use as an npm workspace package

[arbrandes]

Description

Paragon cannot currently be developed as a packages/ workspace of a site built on frontend-base, which is how frontend-base and the MFEs themselves are developed. Three small things get in the way, all fixed here. Closes #4440.

The Makefile called ./node_modules/.bin/babel directly, which does not exist under workspace hoisting. It now resolves babel from PATH, the way the tsc line above it already does.

A clean target plus clean and watch:build scripts give npm run clean:packages and npm run dev:packages something to call, instead of silently no-opping for Paragon.

The checked-in version becomes 0.0.0-dev, like every other Open edX frontend repo, leaving the real version to the release process. 20.18.1 on release-23.x is stale regardless, since semantic-release never commits the bump back.

Compiling the stylesheets takes about three minutes here, against four seconds for everything else, so the build is now split into build-js and build-scss, with the latter keyed on its scss and theme inputs. build still cleans and does everything; watch:build uses build-incremental, which skips the stylesheets unless they changed.

LLM usage notice

Built with assistance from Claude.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.25%. Comparing base (99eda14) to head (e392b0b).
⚠️ Report is 3 commits behind head on release-23.x.

Additional details and impacted files

@@              Coverage Diff              @@
##           release-23.x    #4442   +/-   ##
=============================================
  Coverage         94.25%   94.25%           
=============================================
  Files               242      242           
  Lines              4301     4301           
  Branches            992     1031   +39     
=============================================
  Hits               4054     4054           
+ Misses              243      238    -5     
- Partials              4        9    +5     

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