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

@@ -1,4 +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
+IO.print("abab".indexOf("ab")) // expect: 0

test/string/index_of_invalid_arg.wren

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

test/string/starts_with.wren

@@ -2,4 +2,4 @@ IO.print("abcd".startsWith("cd")) // expect: false
 IO.print("abcd".startsWith("a")) // expect: true
 IO.print("abcd".startsWith("abcd")) // expect: true
 IO.print("abcd".startsWith("abcde")) // expect: false
-IO.print("abcd".startsWith("")) // expect: true
+IO.print("abcd".startsWith("")) // expect: true

test/string/starts_with_invalid_arg.wren

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