Add mocha.opts to option-list filenames

[Alhadis]

Description

This PR adds mocha.opts to the list of filenames recognised as generic option-lists.

Ideally, this really should've been included in #6088, but because mocha.opts files have been deprecated by Mocha for some time, I blindly assumed they wouldn't meet the usage requirements (hence, I didn't think to check). However, a code search for path:**/mocha.opts yields ~9,200 results, which likely spans 200 unique users.

Checklist:

• I am associating a language with a new filename
  • The new filename is used in hundreds of repositories
    ~9,200 repositories
  • I have included a real-world usage sample:
    • Source: hackoregon/civic/packages/2018/mocha.opts
    • License: MIT
  • I have included a change to the heuristics

[Alhadis]

@lildude Low-hanging fruit.