Improve & document NPM package addition process

[StyleT]

At this moment addition of the new NPM package to Lexical monorepo requires repetitive updates to ~10 different files. On top of that Meta specific WWW build requires separate changes done to internal codebase in order to keep it running.

Example: In #5861 we reverted #5830 that added @lexical/devtools-core package. Later it was re-added in #5864 with fixes done to the build system.

Scope:

• Document steps necessary to add new NPM package to Lexical monorepo. Specifically:
  • PR description format to highlight new package addition
  • Changes to the shared files necessary
  • Merge procedure
• Reduce number of changes needed to add new package

[etrepum]

I'd be happy to work on this if you want to delegate, I have been touching the build and documentation a lot lately. See also #5858 and #5850

[etrepum]

I think if we pull out the code I have in those two PRs into shared modules we can use that across the board to handle basically all of the hard-coded mess across the build scripts

• Discover entry points for all non-private packages - this would need some extra stuff to deal with @lexical/react which has multiple entry points and no barrel file (which is basically already in the below script)
• Discover entry points for lexical/react

[StyleT]

@etrepum feel free to jump on it! I can later add Meta specific WWW build steps

[etrepum]

@StyleT will do, if you can get those PRs approved/merged (or at least one of them?) it would make it a little easier for me since AFAIK you can only stack on top of at most one of them as a base branch with the github workflow