feat: support GQL MATCH and graph patterns

[apstndb]

This PR adds support for the GQL MATCH statement:

• [OPTIONAL] MATCH statements
• Full graph patterns
• First-class standalone graph-pattern parsing through both
  memefish.ParseGQLGraphPattern and (*Parser).ParseGQLGraphPattern
• Typed recovery through ast.GQLGraphPatternNode: valid input returns
  *ast.GQLGraphPattern, while malformed input returns a safe
  *ast.BadGQLGraphPattern together with the parse error
• gql and gql_graph_pattern support in tools/parse

The recovered graph-pattern node is safe to unparse, inspect positions, and
walk. It preserves malformed tokens materialized by the lexer, including
first-token string-literal errors. Recovery for a whole GQL graph query remains
separate and is tracked by #383.

Validation

• make ci
• Checked against the Cloud Spanner graph-pattern reference, current
  google/googlesql grammar and parser tests, and the embedded Spanner
  emulator:
  • path variables compose with path search prefixes
  • prefix-like non-reserved keywords can be path variable names
  • edge-only subpaths such as (->) are accepted
  • empty property filters such as (n {}) are syntax errors

Related Issues

• Part of Split GQL related PRs #268
• Related to Missing BadGQLGraphQuery #383

[makenowjust]

Thanks for implementing a big feature!