Predefine QB64PE as $LET value

[electric-socket]

Is your feature request related to a problem? Please describe.
Because someone may be using features exclusive to QB64 Phoenix Edition, there should be a compile-time variable, i.e. a $LET variable QB64PE defined, just as one can check for a correct version of the compiler.

Describe the solution you'd like
Have QB64PE as a predefined $IF/$LET compile-time variable.

Describe alternatives you've considered
Short of the compiler choking on a construct when compiling, there's no way to be certain a particular code works with QB64PE than to have it crash[1], if, for example, QB64 level code is used in a main file but include files use QB64PE-specific features. Even checking version numbers might not be sufficient if the original QB64 has new feature updates and had a version number similar to QB64PE.

[1] "Crash" in this context means "IDE chokes and red flags it."