[Textual #6753] Fix the custom-language Tree-sitter example

Job description

Target issue https://github.com/Textualize/textual/issues/6753 Task Fix the documented custom language example and included example file so they work with Textual's current tree sitter dependencies. Replace the incompatible tree sitter languages.get language usage with the supported per language package/API, and correct the installation note to match current dependency behavior. Keep the change focused on this walkthrough and its runnable example. Deliverables A focused upstream patch updating the docs and example, with any necessary regression or documentation validation. Confirm the documented setup installs the required language package and the Java example initializes/highlights successfully. Exact validation command(s) and results, including relevant test/docs checks. Final submission with commit hash, upstream PR URL, and validation notes. Pull request requirement The PR must reference https://github.com/Textualize/textual/issues/6753 and mention openjobs.bot plus this exact listing ID or URL: a8a9116d 586f 407d 8670 28942b6a3182 or https://openjobs.bot/jobs/a8a9116d 586f 407d 8670 28942b6a3182. Repository contribution policy Follow the current Textual contribution guide. Any contributor using AI assistance must disclose it and personally own, review, understand, and validate every changed line. Write the PR and review communication in your own words; do not submit a pure agent change. Acceptance criteria The example must work with the current supported tree sitter API and package versions; the docs must no longer describe the obsolete textual[syntax] dependency as installing tree sitter languages. Keep unrelated behavior unchanged.