Adding more user friendly checks

[Dashboy1998]

Context

• Over all adds more friendly outputs for errors like those found in #14 & #198
• Currently the only output message with an exit is if you start with PUID or PGID set to 0.
• Adds check for the existence and permissions of directories/files that are needed.
• Makes UPDATE_ON_BOOT, COMMUNITY, MULTITHREADING, BACKUP_ENABLED, AUTO_UPDATE_ENABLED case insensitive
• Checks if an expected boolean is indeed a boolean

In the future I plan to add check for integers, floats, and if the port is valid.

Choices

• Disable container restart
• Using functions as it's checked many times.

Test instructions

Start the container after each step.

#	Action Outside Container	Expected Output
1	chmod 444 palworld/	/palworld/ is not writable.
2	chmod 644 palworld/	/palworld/ is not executable.
3	chmod 755 palworld/	Server Starts
4	chmod 755 palworld/PalServer.sh	Server Starts
5	chmod 000 palworld/PalServer.sh	./PalServer.sh is not readable.
6	chmod 444 palworld/PalServer.sh	./PalServer.sh is not executable.
7	rm -f palworld/PalServer.sh & Start with UPDATE_ON_BOOT=false	./PalServer.sh does not exist, Try restarting with UPDATE_ON_BOOT=true
8	chmod 444 palworld/Pal/Saved/Config/LinuxServer/PalWorldSettings.ini	/palworld/Pal/Saved/Config/LinuxServer/PalWorldSettings.ini is not writable.
9	chmod 755 palworld/Pal/Saved/Config/LinuxServer/PalWorldSettings.ini	Server Starts
10	Test case insensitive boolean values	Server starts
11	Test none true/false strings in place of boolean values	Server starts, outputs that the variable was not a boolean

Checklist before requesting a review

• [x] I have performed a self-review of my code
• [x] I've added documentation about this change to the README.
• [x] I've not introduced breaking changes.