qmk find: usability improvements

[fauxpark]

Description

• Force rules.mk files to be read as UTF-8 (default is CP1252 on Windows, causes issues with Unicode characters)
• qmk find: Add --print argument to print the value of the given key(s) for each matched target
• qmk find: Reworked the filtering a little to add support for length(key, value), contains(key, value), and absent(key). Could probably be refactored further, maybe add !=?

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).