enabled full typescript sourcemaps (with breakpoints and everything!)

this will make debugging both jlab core and extensions much easier inthe future

Author: telamonian

tsconfig.json

@@ -5,6 +5,7 @@
     "declaration": true,
     "esModuleInterop": true,
     "incremental": true,
+    "inlineSources": true,
     "jsx": "react",
     "module": "esnext",
     "moduleResolution": "node",
@@ -15,6 +16,8 @@
     "preserveWatchOutput": true,
     "resolveJsonModule": true,
     "rootDir": "src",
+    "sourceMap": true,
+    "sourceRoot": "./@jupyterlab/git/src",
     "strict": true,
     "strictNullChecks": false,
     "target": "es2017",