chore: rename test files to snake_case and update string class docs

Rename indexOf.wren and startsWith.wren test files to index_of.wren and
starts_with.wren respectively, fix missing trailing newlines, update
string class documentation to clarify UTF-8 storage and move the
note about incorrect UTF-8 handling in the index operator, and
remove unnecessary parentheses in string_indexOf native return.

test/string/index_of.wren

@@ -0,0 +1,4 @@
+IO.print("abcd".indexOf("cd")) // expect: 2
+IO.print("abcd".indexOf("a")) // expect: 0
+IO.print("abcd".indexOf("abcde")) // expect: -1
+IO.print("abab".indexOf("ab")) // expect: 0