[Implement sniff] Verify that the i18n functions are used properly

[jrfnl]

Rule type:

Error

Rule:

From what I can see, there are actually 5 distinct i18n related rules which may need sniffs - this issue covers the fifth item on the list.

1. All theme text strings are to be translatable. (Check that all text strings are translated and in the same language #33)
2. Include a text domain in style.css ([New Sniff] Check that style.css contain a text-domain entry. #34)
3. Use a single unique theme slug – as the theme slug appears in style.css. If it uses a framework then no more than 2 unique slugs. ([Implement Sniff] Check that only one - or at most two - text-domains are used in the theme #35)
4. Can use any language for text, but only use the same one for all text. (Check that all text strings are translated and in the same language #33)
5. Correct usage of the WP i18n functions.

Refs:
https://make.wordpress.org/themes/handbook/review/required/#language
https://make.wordpress.org/themes/handbook/review/required/theme-check-plugin/#additional-checks

There is an existing sniff within WPCS which has extensive coverage in checking whether the i18n functions are used correctly.

Theme check file covering this rule:

https://github.com/Otto42/theme-check/blob/master/checks/i18n.php
https://github.com/Otto42/theme-check/blob/master/checks/textdomain.php

To do:

• Add existing WordPress.WP.I18n sniff to the ruleset.
• Maybe add some more information to the handbook about what the correct usage is of the i18n functions ?

[grappler]

The theme review handbook should only contain the requirements. The documentation for i18n for themes can be found in the theme development handbook https://developer.wordpress.org/themes/functionality/internationalization/

[grappler]

Closing as the PR is merged