Add VS Code config for IDE debugging of tests

This commit adds a launch.json to enable easy IDE debugging of either
Demitasse or (coming soon in js-temporal#41) Test262 tests.

For Test262, there are two different configs:
1) Only debug the subset of tests shown in launch.json. Edit launch.json
   to debug other tests. Warning: commenting out the TESTS env var
   will debug all tests but that will take 10+ mins to run.
2) Only debug the test file currently open in the editor.

.gitignore

@@ -2,5 +2,6 @@ node_modules/
 dist/
 tsc-out/
 .eslintcache
-.vscode/
+.vscode/*
+!.vscode/launch.json
 *.tgz

.npmignore

@@ -3,3 +3,4 @@ test
 types
 tsconfig.json
 *.sh
+.vscode

.vscode/launch.json

@@ -0,0 +1,50 @@
+{
+  "version": "0.2.0",
+  "configurations": [
+    {
+      "name": "Test262 (edit subset in launch.json)",
+      "request": "launch",
+      "type": "node",
+      "runtimeExecutable": "npm",
+      "runtimeArgs": ["run", "test262"],
+      "console": "integratedTerminal",
+      "internalConsoleOptions": "neverOpen",
+      "autoAttachChildProcesses": true,
+      "env": {
+        // Replace the glob pattern below with tests you want to debug.
+        // Comment out to run all tests, but will be very slow!
+        "TESTS": "PlainYearMonth/prototype/subtract/*.js",
+        // Extends timeouts to 1 hour (from 10 seconds default) so you can
+        // debug a test stopped at a breakpoint before the runner kills it.
+        "TIMEOUT": "3600000"
+      }
+    },
+    {
+      "name": "Test262 (current file)",
+      "request": "launch",
+      "type": "node",
+      "runtimeExecutable": "npm",
+      "runtimeArgs": ["run", "test262"],
+      "console": "integratedTerminal",
+      "internalConsoleOptions": "neverOpen",
+      "autoAttachChildProcesses": true,
+      "env": {
+        // Only test the currently-opened file in the IDE
+        "TESTS": "${file}",
+        // Extends timeouts to 1 hour (from 10 seconds default) so you can
+        // debug a test stopped at a breakpoint before the runner kills it.
+        "TIMEOUT": "3600000"
+      }
+    },
+    {
+      "name": "Demitasse tests",
+      "request": "launch",
+      "type": "node",
+      "runtimeExecutable": "npm",
+      "runtimeArgs": ["test"],
+      "console": "integratedTerminal",
+      "internalConsoleOptions": "neverOpen",
+      "autoAttachChildProcesses": true,
+    },
+ ]
+}