chore: simplify build with npm workspaces

[refactorthis]

Description

• simplified build process with npm workspaces
• updated GH workflows to use new npm scripts
• simplified caching in GH workflows

Notes:

• I suggest that we move the packages into packages/* to fit monorepo convention, however this is a large change, so I have not made it here.
• I notice other utility packages in the packages folder were not being built as a part of the install script, or GH workflow, so have not added them to monorepo, I think they should probably be in there - code owners to clarify.

Checklist

• [x] The base branch of this PR is dev, rather than main
• [x] The relevant docs, if any, have been updated or created

Testing

GH workflows were tested with act, however not end-to-end on GH. Please pay particular attention to reviewing these files.

[sestinj]

@refactorthis this is really great. I made a couple of updates to get the build working, but am still running into this:

~/D/continue => npm run prepackage -w extensions/vscode

> [email protected] prepackage
> node scripts/prepackage.js

[info] Using target:  darwin-arm64
[info] Packaging extension for target  darwin-arm64
npm ERR! Cannot read properties of null (reading 'package')

npm ERR! A complete log of this run can be found in: /Users/natesesti/.npm/_logs/2024-09-29T03_29_06_580Z-debug-0.log
Error executing command 'npm install -w extensions/vscode':  ,,npm ERR! Cannot read properties of null (reading 'package')

npm ERR! A complete log of this run can be found in: /Users/natesesti/.npm/_logs/2024-09-29T03_29_06_580Z-debug-0.log

npm ERR! Lifecycle script `prepackage` failed with error: 
npm ERR! Error: command failed 
npm ERR!   in workspace: [email protected] 
npm ERR!   at location: /Users/natesesti/Desktop/continue/extensions/vscode 

If you could help double check the prepackage.js + prepackage-cross-platform.js files, it's the only remaining blocker to merging

[refactorthis]

@sestinj - I've made changes to the build scripts. ran as much as I could to verify, can you check if I've missed anything? I haven't run the gradle build - worth double checking that one.

[sestinj]

Getting really close! Everything looks fine for me except a few odd type errors that are coming up upon npx tsc inside of extensions/vscode (and so also when starting the debugger). May get to these soon, but as usual if you have the interest that could speed things up