Replace acceptUnsupportedGrammar with conditional comments

[nene]

The acceptUnsupportedGrammar option doesn't really work well.

Should instead take another approach to allow skipping bunch of code. One option would be comments like this:

SELECT * FROM foo;
/* sql-parser-cst-disable */
CREATE TYPE float8_range AS RANGE (subtype = float8, subtype_diff = float8mi);
/* sql-parser-cst-enable */
CREATE TABLE tbl (pos float8_range NOT NULL);

This approach still has the issue that you would need to comment-out the whole statement, like the following would not work:

CREATE TYPE float8_range
  /* sql-parser-cst-disable */
  AS RANGE (subtype = float8, subtype_diff = float8mi);
  /* sql-parser-cst-enable */

But at least this would allow you to always disable the parsing for a statement, which the acceptUnsupportedGrammar doesn't achieve.