chore: Initial GHA workflow (#1)

This implements an initial GitHub Actions workflow for us - to be run on PRs and on `main` commits.

This just implements a really simple `style/fmt` check - running `prettier` on the `README.md`.

I assumed we'll stick with using a top-level `Makefile` for commands like in `m` and `link` - but open to alternatives, too!

Since I was adding a `package.json` and `node_modules` for this, I realized we were missing `.gitignore`s, so I added some a subset of the ignore files from `coder/m`

package.json

@@ -0,0 +1,13 @@
+{
+  "name": "coder-v2",
+  "description": "Coder V2 (Workspaces V2)",
+  "repository": "https://github.com/coder/coder",
+  "private": true,
+  "scripts": {
+    "format:check": "prettier --check '**/*.{css,html,js,json,jsx,md,ts,tsx,yaml,yml}'",
+    "format:write": "prettier --write '**/*.{css,htmljs,json,jsx,md,ts,tsx,yaml,yml}'"
+  },
+  "devDependencies": {
+    "prettier": "2.5.1"
+  }
+}