add coverage support

Author: bobzhang

.gitignore

@@ -141,4 +141,5 @@ vendor/ocaml
 jscomp/.lsp
 .vscode/launch.json
 tracing*.json
-darwin/bsc
+darwin/bsc
+.nyc_output

package.json

@@ -1,6 +1,7 @@
 {
   "devDependencies": {
-    "mocha": "^6.2.2"
+    "mocha": "^6.2.2",
+    "nyc": "^15.0.0"
   },
   "bin": {
     "bsb": "bsb",
@@ -13,7 +14,8 @@
     "test-theme": "node scripts/ciTest.js -theme",
     "test-bsb": "node scripts/ciTest.js -bsb",
     "test-ocaml": "node scripts/ciTest.js -ounit",
-    "postinstall": "node scripts/install.js"
+    "postinstall": "node scripts/install.js",
+    "coverage" : "nyc --timeout=3000 --report=html mocha jscomp/test/*test.js && open ./coverage/index.html"
   },
   "name": "bs-platform",
   "version": "7.3.0-dev.1",