Lexer: define varsym without predicates (#22201)

Before this patch, the varsym lexing rules were defined as follows:

	<0> {
	  @varsym / { precededByClosingToken `alexAndPred` followedByOpeningToken } { varsym_tight_infix }
	  @varsym / { followedByOpeningToken }  { varsym_prefix }
	  @varsym / { precededByClosingToken }  { varsym_suffix }
	  @varsym                               { varsym_loose_infix }
	}

Unfortunately, this meant that the predicates 'precededByClosingToken' and
'followedByOpeningToken' were recomputed several times before we could figure
out the whitespace context.

With this patch, we check for whitespace context directly in the lexer
action:

	<0> {
	  @varsym { with_op_ws varsym }
	}

The checking for opening/closing tokens happens in 'with_op_ws' now,
which is part of the lexer action rather than the lexer predicate.