ci: commit-lint stricter footer policy

@KevSlashNull found a workaround in our commit lint CI job:

It's possible to create commit messages that don't have an empty line after the title if we pretend that the second line is a footer (starting with BREAKING CHANAGE: string).

The solution is to enforce the footer-leading-blank.

The original discussion is in !231 (closed). Credit goes to @KevSlashNull

Testing the change

I've created a commit with the following commit message:

refactor: test footer leading blank violation
BREAKING CHANGE: footer lead blank violation

The footer-leading-blank rule had the following effect on such commit.

rule setting	commit-lint job results	pipeline
'footer-leading-blank': [1, 'always']	success, because 1 means warning	https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/pipelines/320438131
'footer-leading-blank': [2, 'always']	failure, because 2 means error	https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/pipelines/320437476