✨ feat: deprecation warnings for Less + monorepo chores

[matthew-dean]

What:

• Adds deprecation warnings to Less output during parsing
• Adds quiet option to silence warnings
• Replaces Lerna w/ PNPM because I honestly can't remember how to use Lerna, and PNPM is easier
• Adds contributor block to README.md
• Cleans up some ESLint issues

Why:

• It would be good to let people know of things that might are deprecated / discouraged and may be removed in the future
• Preps for future Less versions

Checklist:

• Documentation
• Added/updated unit tests
• Code complete

[iChenLei]

Run actions/setup-node@v4
Attempting to download 14...
Acquiring 14.21.3 - x64 from https://github.com/actions/node-versions/releases/download/14.21.3-4202774076/node-14.21.3-linux-x64.tar.gz
Extracting ...
/usr/bin/tar xz --strip 1 --warning=no-unknown-keyword --overwrite -C /home/runner/work/_temp/c7e424d2-ee17-4303-ad5c-cd71ce6168e0 -f /home/runner/work/_temp/c488b341-208e-4f[10](https://github.com/less/less.js/actions/runs/13582507538/job/37970966845?pr=4319#step:4:11)-8bd6-e320e2bc1d13
Adding to the cache ...
Environment details
/home/runner/setup-pnpm/node_modules/.bin/pnpm store path --silent
ERROR: This version of pnpm requires at least Node.js v16.14
The current version of Node.js is v14.21.3

So just drop node 14 test ?