Use gh workflow for tests (#41384)

Follow up for: #41304

Splits CI tests (cherry-picks and PRs only for now) into separate
workflows using `gh-workflow`. Includes a couple restructures to
- run more things in parallel
- remove our previous shell script based checking to filter tests based
on files changed, instead using the builtin `paths:` workflow filters


Splitting the docs/style/rust tests & checks into separate workflows
means we lose the complete summary showing all the tests in one view,
but it's possible to re-add in the future if we go back to checking what
files changed ourselves or always run everything.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

---------

Co-authored-by: Conrad <conrad@zed.dev>

script/prettier

@@ -0,0 +1,17 @@
+#!/bin/bash
+set -euxo pipefail
+
+PRETTIER_VERSION=3.5.0
+
+pnpm dlx "prettier@${PRETTIER_VERSION}" assets/settings/default.json --check || {
+    echo "To fix, run from the root of the Zed repo:"
+    echo "  pnpm dlx prettier@${PRETTIER_VERSION} assets/settings/default.json --write"
+    false
+}
+
+cd docs
+pnpm dlx "prettier@${PRETTIER_VERSION}" . --check || {
+    echo "To fix, run from the root of the Zed repo:"
+    echo "  cd docs && pnpm dlx prettier@${PRETTIER_VERSION} . --write && cd .."
+    false
+}