Full common split: composite tsconfigs and target-* modules

- Add composite TypeScript projects for common/primitives and common/runtime
- Decouple primitives from runtime (buffer service interfaces, UnicodeProperties, CellTypes, etc.)
- Move Types.ts to runtime; re-export primitive types from runtime/Types
- Rename src/browser -> src/target-browser, src/headless -> src/target-headless
- Update import paths (target-browser/*, target-headless/*) across repo
- Fix esbuild outbase so common/* and target-* resolve under out-esbuild/
- Point unicode addons at primitives for UnicodeTypes/UnicodeProperties
- Update ARCHITECTURE.md and AGENTS.md

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

addons/addon-progress/webpack.config.js

@@ -26,7 +26,7 @@ module.exports = {
     extensions: [ '.js' ],
     alias: {
       common: path.resolve('../../out/common'),
-      browser: path.resolve('../../out/browser'),
+      browser: path.resolve('../../out/target-browser'),
       vs: path.resolve('../../out/vs')
     }
   },