migrate to pnpm (#609)

* migrate to `pnpm`

* rename `simple-project` fixture packages to remove socket security errors

* add pnpm `dedupe` options

* update release-pr workflow

* update

* update snashots

* tweaks

* tweak

* tweak

* fix missing ci-setup

* tweak

* tweak deps

* update pnpm

* ok, Socket, let's rename them

* update snapshots

* rename again

* fmt

---------

Co-authored-by: Mateusz Burzyński <mateuszburzynski@gmail.com>

__fixtures__/ignored-package/.changeset/config.json

@@ -1,4 +1,6 @@
 {
   "$schema": "https://unpkg.com/@changesets/config@1.3.0/schema.json",
-  "ignore": ["ignored-package-pkg-a"]
+  "ignore": [
+    "changesets-dev-ignored-package-pkg-a"
+  ]
 }