fix: respect language parameter when filtering by itag in chooseFormat

[vlrdimir]

When multiple formats share the same itag but differ in language (e.g., original vs dubbed audio tracks), chooseFormat now correctly filters by the language parameter instead of always returning the first match.

Changes

• Filter by is_original flag when language = 'original'
• Filter by exact language code for specific languages (e.g., 'en-US')
• Fallback to first format when requested language is not available
• Add warning log when language not found (shows available languages)
• Set default language = 'original' to match documented behavior
• Fix audio selection for videos with multiple language tracks

Before this fix

• Videos with multiple audio tracks (original + dubbed) would always
  download the first format with matching itag (usually German dubbed)
• language parameter was ignored when itag was specified
• No warning when requested language was unavailable

After this fix

• Correctly selects original audio when language = 'original'
• Respects specific language codes when provided
• Warns user when requested language is not found
• Maintains backward compatibility with graceful fallback

Example

Video: https://www.youtube.com/watch?v=M_W-dleZXCs
Has 15 audio tracks (12 dubbed + 3 original) all with itag = 140:

• Before: Selected German (DE) dubbed audio (first in list)
• After: Correctly selects English (US) original audio

[iBicha]

Opinion; I would solve this slightly differently
Instead of selecting best candidate, then falling back to a random option, I would retain the same pattern to be strict with format selection, and throwing an error if it's not available.

Then, I would change the API to accept either a single options: FormatOptions, or an array of options objects.
That way, the caller can specify themselves fallback options, loosening their selection criteria in the array.
chooseFormat would loop through the options array, until it finds a matching format. It would still throw an error if no matches are found within all options objects.