Many number literals were not recognized previously, such as
fractions, infinities, hex and complex literals.

This enriches the space of recognized builtins.

Note that this also fixes the space after a builtin being included in
the token.

Previously, #; awaited an S-expression but could not cope with a
simple datum like a number literal. While at it, also support R6RS
brackets for S-expressions.

Highlighting as Keyword.Declaration is consistent with the
RacketLexer.