chore: emit type declaration source map during build (#614)

Emitting type declaration source map allows using "Go to definition" on other sub-package import to go to source code directly, instead of the emitted type declaration files.

Author: pyrocat101

.vscode/settings.json

@@ -5,5 +5,7 @@
         "**/node_modules/*/**": true,
         "test262/**": true,
         "**/test_artifacts/**": true,
+        "**/*.map": true,
+        "**/*.d.ts.map": true,
     }
 }

packages/intl-messageformat/.npmignore

@@ -7,4 +7,5 @@ tests/
 coverage
 *.config.js
 tsconfig.*
-.nyc_output
+.nyc_output
+*.d.ts.map

tsconfig.json

@@ -10,6 +10,8 @@
         "noUnusedLocals": true,
         "noUnusedParameters": true,
         "preserveConstEnums": true,
+        "declarationMap": true,
+        "sourceMap": true,
         "types": [
             "babel__core",
             "babel__generator",
@@ -30,4 +32,4 @@
             "yargs"
         ]
     }
-}
+}