add string library

substring by index or by content

libraries/LIBRARIES

@@ -22,3 +22,4 @@ frequency_distribution_module.xml	Frequency Distribution Analysis	[EXPERIMENTAL]
 make-variables.xml	create variables in program	declare global or sprite-local variables in a script
 json.xml	Deal with JSON data	Turn JSON strings into lists with the listify block, then retrieve data out of them by using the value at key block.
 parallel_module.xml	Parallelization	Run several scripts in parallel and wait until all are done.
+strings.xml	String processing	Extract substrings of a string in various ways