Update files to end with only one newline and update pre-commit config to include "end-of-file-fixer" & "mixed-line-ending"

[jcfr]

References:

• https://github.com/pre-commit/pre-commit-hooks#end-of-file-fixer
• https://github.com/pre-commit/pre-commit-hooks#mixed-line-ending

Many older tools misbehave if the last line of data in a text file is not terminated with a newline or carriage return / new line combination. They ignore that line as it is terminated with ^Z (eof) instead.

Source: https://stackoverflow.com/questions/2287967/why-is-it-recommended-to-have-empty-line-in-the-end-of-a-source-file