refactor: remove lib and move some things around

[ClementTsang]

Description

A description of the change, what it does, and why it was made. If relevant (such as any change that modifies the UI), please provide screenshots of the changes:

A loooooong time ago (wow, it's been 4 years apparently...), I made the brilliant (/s) decision to use both lib.rs and main.rs because I was trying to add tests in the tests folder that needed private access to some functions in the src - and at the time, the only way I could think of doing so was exposing bottom as a library and a binary.

This isn't necessarily bad, but for my use case, it's pretty unnecessary nowadays (since I've moved all my tests back into the source files) and adds complexity in looking for certain things, so it's high time we move things around and remove lib.rs. I also took the time to clean up some weird code I spotted along the way.

Issue

If applicable, what issue does this address?

Closes: #

Testing

If relevant, please state how this was tested. All changes must be tested to work:

If this is a code change, please also indicate which platforms were tested:

• Windows
• macOS
• Linux

Checklist

If relevant, ensure the following have been met:

• Areas your change affects have been linted using rustfmt (cargo fmt)
• The change has been tested and doesn't appear to cause any unintended breakage
• Documentation has been added/updated if needed (README.md, help menu, doc pages, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts
• If relevant, new tests were added (don't worry too much about coverage)

[codecov]

Codecov Report

Attention: Patch coverage is 40.59701% with 199 lines in your changes missing coverage. Please review.

Project coverage is 40.97%. Comparing base (7aa379a) to head (9ae9322).

Files	Patch %	Lines
src/event.rs	14.77%	75 Missing ⚠️
src/main.rs	59.87%	63 Missing ⚠️
src/app.rs	31.14%	42 Missing ⚠️
src/options.rs	35.71%	18 Missing ⚠️
src/app/process_killer.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1477      +/-   ##
==========================================
+ Coverage   40.95%   40.97%   +0.01%     
==========================================
  Files          99       99              
  Lines       17462    17459       -3     
==========================================
+ Hits         7152     7153       +1     
+ Misses      10310    10306       -4     

Flag	Coverage Δ
macos-12	36.34% <4.77%> (+0.01%)	⬆️
ubuntu-latest	42.92% <40.59%> (+0.01%)	⬆️
windows-2019	36.47% <4.79%> (+0.02%)	⬆️

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