Add an option to highlight the first term of each level

[bcc32]

When reading heavily nested data, such as in lisp-data-mode or other sexp-based data formats, the first term is often used for special purposes, such as the key of each entry in an alist. Enabling this feature makes such terms more visually distinctive.

For context, at Jane Street, we use this to improve highlighting of sexp-based config and data files. We hope others might find this feature generally useful as well.

Before:

After: