chore: Add 'keywords.txt'

[sleepdefic1t]

Proposed changes

This PR adds keywords.txt for highlighting library objects in IDE's such as Arduino.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improve a current implementation without adding a new feature or fixing a bug)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Build (changes that affect the build system)
• Docs (documentation only changes)
• Test (adding missing tests or fixing existing tests)
• Other... Please describe:

Checklist

• I have read the CONTRIBUTING documentation
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[ghost]

@air1one @faustbrian - please review this in the next few days. Be sure to explicitly select labels so I know what's going on.

If no reviewer appears after a week, a reminder will be sent out.

[codecov-io]

Codecov Report

Merging #77 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #77   +/-   ##
======================================
  Coverage    96.4%   96.4%           
======================================
  Files          14      14           
  Lines         250     250           
======================================
  Hits          241     241           
  Misses          9       9

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