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.

doc/site/core-library.markdown

@@ -244,7 +244,7 @@ It is a runtime error if `other` is not a number.
 
 ## String Class
 
-An object representing a string that is created via the string literal syntax.
+A string of Unicode code points stored in UTF-8.
 
 ### **contains**(other)
 
@@ -264,8 +264,8 @@ It is a runtime error if `suffix` is not a string.
 
 ### **indexOf(search)**
 
-Returns the index of `search` in the string or -1 if `search` is not a substring
-of the string.
+Returns the index of `search` in the string or -1 if `search` is not a
+substring of the string.
 
 It is a runtime error if `search` is not a string.
 
@@ -291,11 +291,12 @@ Check if the string is not equal to `other`.
 
 ### **[**index**]** operator
 
-Returns a one character string of the value at `index`. This does not handle
-UTF-8 characters correctly.
+Returns a one character string of the value at `index`.
 
 It is a runtime error if `index` is greater than the length of the string.
 
+*Note: This does not currently handle UTF-8 characters correctly.*
+
 ## List Class
 
 **TODO**