Parse and store suffixes on literals.

This adds an optional suffix at the end of a literal token:
`"foo"bar`. An actual use of a suffix in a expression (or other literal
that the compiler reads) is rejected in the parser.

This doesn't switch the handling of numbers to this system, and doesn't
outlaw illegal suffixes for them yet.
Switch numeric suffix parsing to use the new system.

This moves errors and all handling of numeric suffixes into the parser
rather than the lexer.
Adjust Antlr4 lexer to include suffixes.

This makes the formal lexical grammar (more closely) reflect the one
implemented by the compiler.
Update documentation for literal suffixes.

This changes the stated grammar of literals to move all suffixes into
the generic literal production.