Add string 'repeat' subcommand

This feature add the ability to repeat a string a given number of times.
For example: string repeat -n 3 foo

tests/string.out

@@ -63,3 +63,23 @@ missing argument returns 0
 no regexp invert match
 no glob invert match
 1 3
+foofoo
+foofoo
+foofoo
+exit 0
+exit 0
+exit 1
+exit 1
+exit 1
+there is no newline
+there is no newline
+foof
+foofo
+foofoofoo
+foof
+exit 2
+exit 2
+exit 2
+exit 2
+exit 2
+exit 2