Convert to V2 addon

[gilest]

Classic big bundle of deferred maintenance. I've tried to change as little as possible while getting a working build and passing tests.

Attempted to use community blueprints and tools with a particular view to ease of maintenance long-term.

• [x] Package manager -> pnpm
• [x] Project layout -> pnpm workspace (monorepo)
• [x] ember-cli-flash package updated by V2 Addon blueprint
• [x] test-app package updated by Ember CLI app blueprint
  • [x] I also took this to 4.12 to get relatively updated tooling
• [x] Node -> 16 (Although in v2 addons this isn't so important as they are prebuilt)
• [x] Render modifiers -> Local modifier
• [x] Peer deps on @ember/string and ember-modifier

Breaking 💥

• Consuming apps now require ember-auto-import v2
• Node 16 (should we go 18 right away?)
• Dropped Ember "Classic" support
• Dropped Ember 3.24 support. Minimum supported version is now 3.28

Unresolved/maybe breaking

• Type definitions – I've manually placed these in declarations/ and tested this locally
• Releasing will be similar but the RELEASING file will need to be adjusted slightly

Todo soon after

• Migrate to typescript so that declarations can be built automatically with source mapping etc
• Replace and document the automatic (via CLI blueprint) test helper (I've extracted a replacement for this into its own branch, I'll open a PR)