Add validation for Finnish phone numbers

[simison]

Got this from https://en.wikipedia.org/wiki/Telephone_numbers_in_Finland and modified to support 00358 and not only +358 at the beginning of the international format.

Includes tests.

(Lemme know if you want this squashed to one commit.)

[chriso]

Can we cut down the regex at all? It looks larger than all of the other locales combined 😓

[simison]

It does look really long, indeed! Crazy. :-) But then, what's the exact problem with that? (other than visual)

It includes Finnish landline numbers as well, where as the other ones seem to include only mobile numbers.

[chriso]

Ah I see. Mobile phone numbers only then, please. General phone number validation has been requested before, but then the library slowly descends into a crappy version of libphonenumber which is huge.

[simison]

Aight! Valid point.

I'll ping you once I'm done with the change. Gimme some days.

[chriso]

No problem, and thanks!

[chriso]

Any update on this?

[simison]

Edited + squashed + rebased on top of current master.

Other validators don't validate totally valid leading zeros: 00358 instead of +358. Should I remove that bit as well?

I could send another PR to add them to other phone numbers, tho.

[chriso]

That looks good. Thanks again 😄