Hi,
This PR adds support to allow developers to explicitly choose to set environment variables as a literal (ie. wrapped in single quotes).

E.G.
Set-EnvFileVariable -Variable 'VAR4' -Value 'literal$tring' -AsLiteral

Would write the following to the .env
VAR4='literal$tring'

I believe this is an improvement due to:

• Improved DX giving developers the explicit choice of when to use literals when setting/getting env vars
• Allows for special characters in keys/secrets when setting/getting env vars that may have been generated in other systems, which in some cases may not be avoided
• Improves IDE syntax highlighting in .env files when there are special characters (special characters in long strings throw it off)
• Enhances support for all previous flags when using Get-SitecoreRandomString e.g. -DisallowSpecial, -DisallowDollar etc where used

Bonus: Also fixes a bug where the replacement of $ in Set-EnvFileVariable will result in different outcomes depending on if it was an existing variable (inserted via regex) vs a new variable (interpolated with the $'s doubled) in the .env file.