Add gettext support

[ArtyIF]

Dictionary for gettext

I'm not sure how to create new dictionaries for cspell, but for the gettext format (po) at least the following words need to be added:

• msgctxt
• msgid
• msgid_plural
• msgstr
• nplurals (for the Plural-Forms header)

Reference

• gettext - Wikipedia
• GNU gettext utilities: PO Files
• PO and POT files | Working with offline translation files | Community guide on Drupal.org
• What are .PO files?