Introduce parser_errors spec test suite

[karreiro]

Overview

liquid-spec doesn't have many cases covering syntax errors. This PR adds ~2K specs covering nearly every SyntaxError branch across all tags — all additive YAML, no Ruby changes.

Each spec follows this shape:

- name: "assign - missing variable name"
  template: "{% assign = 'hello' %}"
  errors:
    parse_error: "[Liquid syntax error] Syntax Error in 'assign' - Valid syntax: assign [var] = [source]"

How adapters opt in

The suite is opt-in, so existing adapters see nothing new. An adapter that supports strict2 parsing enables the full suite by adding it to their suite list; the existing feature-gating system handles the rest.

Coverage

The specs aim to cover every raise SyntaxError call site in the parser including unguarded parser.consume(type) calls.