Include checking for custom changelog note in pull description when generating changelog.

[nerrad]

When implementing the changelog generation script for the release pull request automation, I didn't include the ability for reading optional custom changelog notes added in a pull description (as used in other scripts like WooRelease, and by other teams in the WooCommerce org).

At first I didn't think it was necessary and that we should focus on making our pull request titles changelog worthy. However, after some discussion with the team, I think it'd be worthwhile to still consider the option given the following reasons:

• It's consistent with what other Woo teams are doing (in particular WooCommerce core) - and also supported by the "official" release script (WooRelease) used by Woo teams for most extensions.
• There's valid argument for git blame/history having more dev centric specific pull request titles given the context. I still personally think there's value in customer centric git history but it's not a sticky point for me, so having the option for variation could be useful.

I'm just dropping in a code snippet here that can serve as a rough base fo re-implementing:

let title;
	if ( /### Changelog\r\n\r\n> /.test( pullRequest.body ) ) {
		const bodyParts = pullRequest.body.split( '### Changelog\r\n\r\n> ' );
		const note = bodyParts[ bodyParts.length - 1 ];
		title = note
			// Remove comment prompt
			.replace( /<!---(.*)--->/gm, '' )
			// Remove new lines and whitespace
			.trim();
		if ( ! title.length ) {
			title = `${ type }: ${ pullRequest.title }`;
		} else {
			title = `${ type }: ${ title }`;
		}
	} else {
		title = `${ type }: ${ pullRequest.title }`;
	}