semconvgen: Add a flag for a path to a custom capitalizations file

[carrbs]

Closes #528

This PR:

• adds a new flag to semconvgen to read custom strings from a plaintext file
• confirms the file is present in the filesystem
• reads the custom strings from the given file (with error handling)
• appends the strings to the static capitalizations slice

[carrbs]

Hi @codeboten 👋 Do you have any thoughts on this? (I misread the CONTRIB file and thought I was supposed to add [WIP] to the title to solicit feedback when I opened the PR a few weeks ago 🤭).

[codecov]

Codecov Report

Attention: Patch coverage is 52.17391% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 60.64%. Comparing base (dbdb565) to head (21a437c).

Files	Patch %	Lines
semconvgen/generator.go	52.17%	9 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #529      +/-   ##
==========================================
- Coverage   66.05%   60.64%   -5.41%     
==========================================
  Files          39       40       +1     
  Lines        1785     1964     +179     
==========================================
+ Hits         1179     1191      +12     
- Misses        461      626     +165     
- Partials      145      147       +2     

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[pellared]

I think that we are not properly handling empty lines (or lines with only whitespaces).

Side note: I would not catch it without seeing the tests 😉

[pellared]

ci/lint failed

[carrbs]

ci/lint failed

@pellared I fixed this here: 2e3b1fe

[pellared]

@carrbs Thanks for working on this👍