func_strings: Add STRBETWEEN function

Adds the STRBETWEEN function, which can be used to insert a
substring between each character in a string. For instance,
this can be used to insert pauses between DTMF tones in a
string of digits.

ASTERISK-29627

Change-Id: Ice23009d4a8e9bb9718d2b2301d405567087d258