As an editor I want to be able to select the right spelling variant already when creating the new Lexeme, not only once the Lexeme has already been created.

Problem:
Special:NewLexeme does not allow you to put in custom spelling variants like de-x-Q1234. We should allow this in addition to the list of selectable spelling variants.

Example:

• de-x-Q1234

Screenshots/mockups:
TBD

BDD
GIVEN
AND
WHEN
AND
THEN
AND

Acceptance criteria:

• custom language codes can be entered for the spelling variant on the Special:NewLexeme page
• input is validated and only the codes allowed on the Lexeme page are accepted on Special:NewLexeme

• • Open questions:**
• How do people even get there? For example when selecting German as the language then the spelling variant input field would not be shown to them.

Notes:

• split out from https://phabricator.wikimedia.org/T298143