github: include /.github/ in ripgrep searches by default

Previously, unless you had some other configuration that impacts
ripgrep, `rg -tyaml uses:` would return zero results. After this
changes, it returns more of what you might expect.

This is because ripgrep ignores hidden files and directories by default.
But arguably, searching `.github` by default is probably what we want.

I do the same thing in ripgrep's repository:
https://github.com/BurntSushi/ripgrep/blob/de4baa10024f2cb62d438596274b9b710e01c59b/.ignore#L1

.ignore

@@ -0,0 +1 @@
+!/.github/