DEV: Include plugin directories in search tooling & IDEs (#29760)

Non-bundled plugins are gitignore'd, but we want them to show up in search tools / IDEs. Adding a `.ignore` file with a negative glob lets us achieve this.

Previously, it was up to individuals to work out how to configure their editor to do this when working on plugins.

Also adds negative matchers for the vscode config files, so they show up in the file picker & search.

.ignore

@@ -0,0 +1,8 @@
+# Non-bundled plugins are gitignore'd, but we want them to
+# show up in search tools / IDEs so we un-ignore them here
+!/plugins/*
+
+# Allow these files to show up in vscode file selector, even
+# though they're not git-tracked
+!/.vscode/settings.json
+!/.vscode/tasks.json