chore(vscode): Add VSCode recommended extensions

  The provided `extensions.json` file is based on the one
[in `mozilla‑central`][mozilla‑central‑vscode‑extensions].

[mozilla‑central‑vscode‑extensions]:
https://hg.mozilla.org/mozilla-central/file/tip/.vscode/extensions.json

Author: ExE-Boss

.gitignore

@@ -39,6 +39,7 @@ capture_webrender/
 
 # VSCode
 .vscode
+!/.vscode
 
 /unminified-js
 

.vscode/extensions.json

@@ -0,0 +1,20 @@
+{
+    // See http://go.microsoft.com/fwlink/?LinkId=827846 to learn about workspace recommendations.
+    // Extension identifier format: ${publisher}.${name}. Example: vscode.csharp
+
+    // List of extensions which should be recommended for users of this workspace.
+    "recommendations": [
+        // Trim only touched lines.
+        "NathanRidley.autotrim",
+        // ESLint support.
+        "dbaeumer.vscode-eslint",
+        // C/C++ language support.
+        "ms-vscode.cpptools",
+        // Rust language support.
+        "rust-lang.rust",
+        // CSS support for HTML documents.
+        "ecmel.vscode-html-css",
+        // IDL language support
+        "mythmon.idl"
+    ]
+}