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_invalid_arg.wren

@@ -0,0 +1 @@
+IO.print("abcd".indexOf(null)) // expect runtime error: Argument must be a string.