feat: add a script to check for PR title and labelling

[grunweg]

The script checks most items from the commit convention, ensures that a feature PR has a topic/area label,
and errors if any two labels are contradictory.

Extracted from #16303. That PR will make this workflow run in CI.

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[github-actions]

PR summary 4a05ec2e85

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

+ checkTitleLabels
+ checkTitleLabelsCLI
+ hasContradictoryLabels
+ main
+ splitAtPos
+ validateTitle

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

[grunweg]

Since the other PR has CI issues, let me extract the validation logic to a separate PR, which can hopefully land earlier.
@bryangingechen @joneugster Would one of you like to re-review? There are lots of tests now, which might speed that up.

[bryangingechen]

Just checking: Lean files in scripts aren't checked by CI, right?

[grunweg]

It depends: syntax linters run on every file importing them, but otherwise CI shouldn't touch them.
This script is not used or touched by CI until a workflow uses it (as in, e.g., the next PR).