lib/helpers: eliminate assumptions about login shells

[gaelicWizard]

Description

Bash loads initialization files on Mac just the same as it does on Linux or WSL. Our previous assumptions were wrong, and my fix was alsö wrong because I made more assumptions!

This patch eliminates the assumptions. Literally just load either the startup file the shell started with, or fall back to ~/.bashrc. Don't check shopt -q login_shell and don't check $OSTYPE or anything else.

Motivation and Context

I broke @NoahGorny's WSL shell. WSL runs shells as login shells, just like on Mac. But, since it loads an entire Linux distribution under the hood, WSL includes workarounds for Bash's weird startup file behaviours and, therefore, my assumption that the startup file for a login must always be ~/.bash_profile (or ~/.profile) was quickly proven wrong. 😞

How Has This Been Tested?

By Noah. 😆

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[gaelicWizard]

@NoahGorny, this works best with #1902, which is where I added $BASH_IT_BASHRC, but falls back to ~/.bashrc

[NoahGorny]

This is much better now!