feat: expose localeProperties property

[iifawzi]

Hi, as mentioned here #916, @rchl decided to expose the current locale object, here's how i tackled it, i hope i did it in the correct way since this's my first contribution!

i think we can move the codeFromLocale function from seo-head to utils-common to use it here also, instead of duplication, if confirmed i will commit the change

[rchl]

• The locales property can also be just an array of locales in which case your code will crash. If that's the case we can just set an empty object.
• Please initialize the new property also around line 263
• We'd need to add it to the documentation. In docs/content/en/api.md (and es version) for sure but ideally also mention it in the documentation of the locales option in options-reference.md.
• At least a single test that checks that this property exists and is set to the correct value should be added.

[iifawzi]

Hi @rchl, i made all the mentioned points, changes are committed.
i added two tests to ensure that the property exists and is set to the correct value, when configuration is empty, and when array of locale objects is used.

[rchl]

Looks good from my side. I've done some small changes, please have a look and let me know if those look good to you.

[iifawzi]

looks awesome from my side, well done!