[Web] Support structural tag for web-xgrammar package, update web to 0.1.18

[CharlieFRuan]

This PR updates the web-xgrammar TypeScript package, mainly adding support for Structural Tag. There are two components to this PR:

1. Address trivial changes to the APIs, including adding any_whitespace and ebnfToGrammarNoNormalization(), and fixing any failed unit tests due to changes in the C++ source (see first commit 6bccf6d)
2. Add structural tag support. Specifically, we export StructuralTagItem, fromStructuralTag(), and compileStructuralTag(). In addition, we add isGrammarAcceptString() for testing. Necessary changes are made to xgrammar_binding.cc

No change is needed outside of the web/ folder.

The structural tag feature is tested with TypeScript unit tests equivalent to test_grammar_matcher_structural_tag.py, and an E2E example in web/example/.

We subsequently publish 0.1.18 npm package https://www.npmjs.com/package/@mlc-ai/web-xgrammar

[Copilot]

Pull Request Overview

This PR adds support for structural tag grammars in the web‑xgrammar package while updating JSON schema conversion and compilation functionality in line with Python improvements.

• Introduces a new StructuralTagItem class and updates tests to validate structural tag grammar handling.
• Adds new C++ binding functions and corresponding TypeScript wrappers for structural tag support.
• Updates JSON schema to EBNF conversion and compilation functions to include an "any_whitespace" parameter.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
web/tests/grammar.test.ts	Updated test cases to include structural tag testing and JSON schema adjustments
web/src/xgrammar_binding.cc	Added new binding functions for structural tag support
web/src/xgrammar.ts	Added StructuralTagItem and updated grammar functions for new parameters and structural tag grammar
web/src/index.ts	Updated exports to include StructuralTagItem
web/example/src/example.ts	Added an example demonstrating structural tag grammar usage

Comments suppressed due to low confidence (1)

web/src/xgrammar.ts:292

• There's a spelling error in the comment: 'corrrespondence' should be 'correspondence'.

 *   *   The corrrespondence of tags and triggers is automatically determined: then if the trigger is encountered, the following output must match one