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.
Replace the empty `core-libraries.markdown` placeholder with a comprehensive `core-library.markdown` documenting Object, Class, Bool, and Fiber classes. Update site template to link to the new page. Tweak SCSS styles: reduce body font to 16px/25px, darken header background with text-shadow, adjust code font size and borders, add left margin to main content paragraphs and pre blocks, and refine link colors. Fix prose in functions.markdown to use "arguments" instead of "parameters" and clarify dynamic dispatch wording.
