Type declarations

[dingo-d]

This PR depends on #101. Once that is merged, this PR should be rebased to the master branch.

The PR adds rules about the type declaration rules and is the continuation of the additions of 'modern' PHP code in the WordPress PHP Coding Standards handbook based on the make post by Juliette Reinders Folmer.

[dingo-d]

I've added all the types currently available that cannot be used in core (still), but since the wording in the alert now excludes core I've reworded the phrase until the minimum version is raised to until the minimum PHP version is ... or higher. Because it didn't really make sense (as there is no mention of WordPress, so where the PHP minimum required version comes from wasn't really clear to me in the sentence context).

I'm open to rewording these statements if they read weird in any way.

[jrfnl]

Gone through it with a fine toothcomb....

[jrfnl]

See inline comments + the remark about the order of information.

[jrfnl]

Approving. There is a stray space in line 425 "code[space][space]new to Core", which I will remove before merging.

Merging this now as @GaryJones is AFK for the next few weeks. If needs be, additional textual tweaks can still be made in a follow up commit.