add getACodepoint to the shared Strings library, and use it in NfaUtils

shared/util/codeql/util/Strings.qll

@@ -63,6 +63,12 @@ string getCodepointAt(string s, int i) {
     )
 }
 
+/**
+ * Gets any unicode character that appears in `s`.
+ */
+bindingset[s]
+string getACodepoint(string s) { result = s.codePointAt(_).toUnicode() }
+
 /**
  * Gets the number of unicode codepoints in `s` not counting unpaired surrogates.
  */