Revert "Remove the separator characters construct"

This reverts commit 5cd07648fd.

The separators construct is useful as an optimization. It turns out that
constructing a node for every chunk of whitespace in a document causes a
significant performance regression.

Conflicts:
	src/compiler/build_tables/build_lex_table.cc
	src/compiler/grammar.cc
	src/runtime/parser.c

src/runtime/stack.h

@@ -25,7 +25,6 @@ void ts_stack_push(TSStack *stack, TSStateId state, TSTree *node);
 TSStateId ts_stack_top_state(const TSStack *stack);
 TSTree *ts_stack_top_node(const TSStack *stack);
 size_t ts_stack_right_position(const TSStack *stack);
-TSTree **ts_stack_pop_extras(TSStack *, size_t *);
 
 #define TS_STACK_FROM_TOP(stack, entry, index)               \
   size_t index = stack.size - 1;                             \