New release workflow triggered by the merge of a PR changing

IS_DEVELOPMENT_BUILD to false

This is more streamlined than the previous approach, and works better
for a world where 1 person isn't doing all the work.

Now, the flow is simpler:
- Do changes (e.g. protocol update), changelog & set IS_DEVELOPMENT_BUILD to false all in a single PR, which can be squash-merged if desired
- Once the PR is merged, a draft release will be prepared
- RestrictedActions will automatically set IS_DEVELOPMENT_BUILD back to
  true and bump the version
- Tag will be created when the release is published

Previously, multiple PRs might be needed, and the PR containing the
release changelog couldn't be squash-merged. Manual intervention was
also required to create a tag and prepare a release.

This PR also includes new CI checks to check for basic errors like
forgotten changelog files to ensure changelog links work correctly.

Note: Only PRs from PMMP Team members with **write** access to the
repository can trigger release generation. Random people cannot trigger
release generation by sending PRs.

phpstan.neon.dist

@@ -40,6 +40,7 @@ parameters:
 			- build/php
 	dynamicConstantNames:
 		- pocketmine\VersionInfo::IS_DEVELOPMENT_BUILD
+		- pocketmine\VersionInfo::BUILD_CHANNEL
 		- pocketmine\DEBUG
 		- pocketmine\IS_DEVELOPMENT_BUILD
 	stubFiles: