FB2 writer: Add support for "lang" metadata

[link2xt]

I need this when converting from Muse to FB2. Muse uses #lang according to its manual and FB2 uses <lang> tag, and EPUB writer also uses lang metadata, so everything is compatible.

[mb21]

Pandoc's lang metadata field is in BCP 47 (e.g. en-GB), which is basically the ISO 639 language code (e.g. en) followed by an optional part indicating the regions/script etc.

Not exactly sure what the Muse people mean with "The language code of the document (2 or 3 letters)", but sounds like the dash and everything following it should be stripped.

[link2xt]

@mb21 What Muse uses is irrelevant here, but my Muse documents use something like "ru" instead of "ru-RU". It is really unspecified, directives in Muse are free form, so I am not going to do anything with them in my Muse reader (see PR #3620).

What really matters is what FB2 standard says, and looks like it uses ISO 639.

So I should probably try to convert BCP 47 to ISO 639 when writing lang to FB2.

[link2xt]

Also related: https://bugs.launchpad.net/calibre/+bug/990026

[link2xt]

Looks like FB2 documents actually use "en" and no "en-US" or "en-GB", so just stripping everything up to "-" is enough. Fixed it.

[jgm]

Thanks!