Build tooling and packaging updates

[microbit-matt-hillsdon]

• Remove DeviceVersion exported const enum in favour of a union for this reason (breaking change due for code like DeviceVersion.V1 but not expected to have wide impact. The values are unchanged.).
• Remove UMD build. Stop offering a browser oriented bundle.
  • If we know of any real-world script-tag-only users then I'm open to reinstating this but I'm really dubious it's worth it.
• CJS now uses dependencies rather than bundles to aid use alongside microbit-connection (avoiding duplicating nrf-intel-hex in that scenario).
• Fix ES module so it works with Node's ES module support (exports in package.json, .js file extensions on imports)
• Emit more modern JavaScript (ES2021, consistent with microbit-connection).
• Consolidate documentation into typedoc
  • Move the markdown content into typedoc's build so we don't have separation and simplify
  • Drop the unmaintained theme
  • Move to GHA publishing it to GitHub pages
  • This is now simple enough I can imaging rolling it out to other libraries
• Update linting, TypeScript etc.
  • Intending to port most of this to typescript-library-starter, but hopefully using NodeNext there.
• Stuck on "node10" TypeScript bundleResolution for the same reason as Add cjs build alongside esm microbit-connection#51

Other than DeviceVersion there's no code-level behaviour change but it's likely also a breaking change due to the ES2020 emit and the packaging changes so bumping version to 0.10.0. I think it should generally be easier to consume in modern projects while still working in CJS/webpack 5 type setups.

[microbit-robert]

This now looks good to me pending any comments from @microbit-carlos

[microbit-carlos]

No additional comments from my side. Looks like a good simplification of the docs and packaging (even if we lose the browser bundle).