Add support for required WP and PHP versions to themes

[flixos90]

As a follow-up to #40934 and accompanying the related meta ticket ​https://meta.trac.wordpress.org/ticket/3718, themes should be allowed to specify minimum required WordPress / PHP versions.

The scope of this ticket is solely to add support for these two headers to WP_Theme, so that they are parsed correctly. Catching unmet version requirements and preventing theme installations/updates/activations should be part of separate follow-up tickets.

This ticket needs to be tackled in alignment with the above meta ticket, in terms of which names to use for the two new headers.

[flixos90]

44592.diff​ adds support for a Requires header (for required WP version) and a Requires PHP header (for required PHP version) to WP_Theme. While we could go with Requires WP, I thought it would make sense to align the names with the names of the plugin headers, so people can remember them easily for both plugins and themes.

Going forward though, let's have the discussion on header names in the meta ticket ​https://meta.trac.wordpress.org/ticket/3718, and adjust the patch here accordingly as necessary.

[afragen]

See #46938

[afragen]

I think we should use Requires WP as it is more descriptive and aligns well with Requires PHP.

[afragen]

Looks like we should be using Requires at least as that is what was committed in r45546

[afragen]

Change to Requires at least

[afragen]

Patch updated to use Requires at least.

I'm working on testing themes for WordPress and PHP compatibility. Early commit of this ticket is going to be needed or it will end up being in subsequent patches.

[SergeyBiryukov]

In 47145:

Site Health: Add support for required WordPress and PHP versions to themes.

Follow-up to [45546] for plugins.

Props flixos90, afragen.
Fixes #44592. See #40934, ​#meta3718.

[audrasjb]

This change appears in the miscellaneous changes dev note: ​https://make.wordpress.org/core/2020/02/26/miscellaneous-developer-focused-changes-in-wordpress-5-4/