Fix use of build_pure_c_string in treesit.c

Fix use of build_pure_c_string in treesit.c

This is brought up in bug#60691.  build_pure_c_string should only be
used in places such as syms_of_treesit, which are called just once,
during dumping.

* src/treesit.c (Vtreesit_str_libtree_sitter):
(Vtreesit_str_tree_sitter):
(Vtreesit_str_dot):
(Vtreesit_str_question_mark):
(Vtreesit_str_star):
(Vtreesit_str_plus):
(Vtreesit_str_pound_equal):
(Vtreesit_str_pound_match):
(Vtreesit_str_pound_pred):
(Vtreesit_str_open_bracket):
(Vtreesit_str_close_bracket):
(Vtreesit_str_open_paren):
(Vtreesit_str_close_paren):
(Vtreesit_str_space):
(Vtreesit_str_equal):
(Vtreesit_str_match):
(Vtreesit_str_pred): New variables.

(treesit_load_language):
(Ftreesit_pattern_expand):
(Ftreesit_query_expand):
(treesit_eval_predicates): Use new varaibles.

(treesit_check_buffer_size):
(treesit_compose_query_signal_data):
(treesit_check_range_argument):
(Ftreesit_parser_set_included_ranges):
(treesit_predicate_capture_name_to_node):
(treesit_predicate_equal):
(treesit_predicate_match):
(treesit_predicate_pred): Use build_string for signal message.

(syms_of_treesit): Initialize new variables.