Kotlin: Special-case String.charAt naming

In the Kotlin universe this is called `get` so that Kotlin programmers can use the `[]` operator on `String`s.

java/ql/test/kotlin/library-tests/string-charat/Test.java

@@ -0,0 +1,5 @@
+public class Test {
+
+  public char f(String s) { return s.charAt(0); }
+
+}

java/ql/test/kotlin/library-tests/string-charat/test.expected

@@ -0,0 +1,2 @@
+| Test.java:3:36:3:46 | charAt(...) |
+| test.kt:2:20:2:23 | charAt(...) |

java/ql/test/kotlin/library-tests/string-charat/test.kt

@@ -0,0 +1,2 @@
+
+fun f(x: String) = x[0]

java/ql/test/kotlin/library-tests/string-charat/test.ql

@@ -0,0 +1,4 @@
+import java
+
+from MethodAccess ma
+select ma