fromfilename: improvements & fixes

[henry-oberholtzer]

Description

This PR aims to eventually improve the usability and configuration of the fromfilename plugin to encompass more file name
patterns and folder names, which can contain usable information.

Goals:

• Full typing
• Increased test coverage with more edge case handling
• Make sure single files can also be checked for information
• Precompile regex expressions
• Expanded patterns from track information FromFilename Plugin don't work at all #5597
• Default patterns with better track index parsing fromfilename: Add filename matching regex for more forms of leading numerals #541
• Default behavior extracts information from the folder name (album artist, year, album title) fromfilename: Guess more fields from parent directory name #1554
• Allow users to contribute additional search patterns & fields fromfilename: Configurable extraction patterns #1400
• Ignore parent folder configuration option
• Consider configuration option to allow updating information when "Use-As-Is" is selected. fromFilename Plugin: Does not update tags with AsIs actions. #5413

Notes:

• Removed the "tag" field from the plugin, as its use and applications were vague, undocumented, and untested.

Housekeeping

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[codecov]

Codecov Report

❌ Patch coverage is 99.56897% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 68.56%. Comparing base (afc26fa) to head (4672ee0).
⚠️ Report is 17 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/fromfilename.py	99.56%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6259      +/-   ##
==========================================
+ Coverage   68.22%   68.56%   +0.33%     
==========================================
  Files         138      138              
  Lines       18792    18983     +191     
  Branches     3167     3190      +23     
==========================================
+ Hits        12821    13015     +194     
+ Misses       5298     5297       -1     
+ Partials      673      671       -2     

Files with missing lines	Coverage Δ
beetsplug/fromfilename.py	99.57% <99.56%> (+6.92%)	⬆️

... and 3 files with indirect coverage changes

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