Add recursion limit for string expansions

Unlike macros, REPTs and INCLUDEs, this recursion depth is independent.
This is intentional, because string expansions work very differently.

While it's easy to know when a string expansion begins, checking where it
ends is much more complicated, since the expansion's contents are simply
injected back into the lex buffer. Therefore, the depth has to be checked
after lexing took place.
Because of this, the placement of the expansion end check is somewhat
haphazard, but I think it's good. While I have no certainty, all tests
ended with all expansions properly ended, and I couldn't find any pitfalls.

Finally, `pCurrentStringExpansion` has been made global so error printing
can use it to tell the user if an error occurred inside of an expansion.

include/asm/fstack.h

@@ -35,7 +35,7 @@ struct sContext {
 	uint32_t nREPTBlockSize;
 };
 
-extern unsigned int nMaxFileStackDepth;
+extern unsigned int nMaxRecursionDepth;
 
 void fstk_RunInclude(char *tzFileName);
 void fstk_RunMacroArg(int32_t s);