feat: string funcs, assert true/false

[FAReTek1]

this pr adds a couple of functions in the string.gs file.
this also adds assert true/false procs as assert_t and assert_f, using them in the string test file

these are going to be loosely based on the python string methods and the python string package

Note

Maybe slice_step should just be renamed to slice, and the original slice function should be removed, and then add a default argument for the step?

[FAReTek1]

close?