Allow program to be run from outside its own directory + other updates

[Nel-S]

• Add the path used to open the program from to all relative paths, so program doesn't create new folders in a directory outside the one it's located within
• Replace sys.exits with exception raising (see Improvements for programmatic usage #55)
• Replace assertion (which can be compiled out) with unskippable test + exception
• Tweak url['..._mappings'] code so correct exception is raised (Python stack trace when I run DecompilerMC with an older Minecraft version without client-mappings #50)
• Standardize pathlib usage
• Some code restructuring to reduce the amount of indentation
• Add some typing
• Grammar fixes in code outputs/errors + significantly copyedit Readme (but no new information was really added)
• Add TODOs to main.py to mark areas for future improvement

[hube12]

This does look good to me, I will need to check that this can be run by double clicking and not exiting the windows without you being able to read the message (the main purpose of the press a key then sys.exit(1))