Fix addon and dist builds after path mapping removal

Import shared core from ../../../out/common|browser in addons so tsc
and runtime agree. Enable preserveConstEnums so const enums remain in
compiled out/ for addon consumers.

Build xterm esbuild before addons so out-esbuild exists for parallel
addon jobs. Verified: npm run setup, esbuild-demo-client, package.

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

src/tsconfig-library-base.json

@@ -5,6 +5,7 @@
     "strict": true,
     "declarationMap": true,
     "experimentalDecorators": true,
-    "downlevelIteration": true
+    "downlevelIteration": true,
+    "preserveConstEnums": true
   }
 }