feat: add refactorex

[RefactorEx](https://github.com/gp-pereira/refactorex) is an LSP server
that allows for refactoring Elixir code. It adds the ability to rename
symbols via `vim.lsp.buf.rename()` and various code actions.

Right now it uses an intermediary repository,
https://github.com/synic/refactorex-mason to download the sourcecode,
patch it to support the stdio transport, and create a launch script to
use the stdio transport. Hopefully in the future this intermediary
repository won't be necessary, once refactorex itself supports stdio
(gp-pereira/refactorex#19).

packages/refactorex/package.yaml

@@ -0,0 +1,20 @@
+---
+name: refactorex
+description: Language server allowing for refactoring Elixir code
+homepage: https://github.com/gp-pereira/refactorex
+licenses:
+  - MIT
+languages:
+  - Elixir
+categories:
+  - LSP
+
+source:
+  # renovate:datasource=github-tags
+  id: pkg:github/synic/[email protected]
+  build:
+    run: ./build.sh
+    bin: exec:refactorex-release/bin/start-stdio
+
+bin:
+  refactorex: "{{source.build.bin}}"