feat(tree-sitter): add tree-sitter devcontainer feature

[pocky]

Summary

• Add tree-sitter as a new devcontainer feature, enabling consistent CLI installation and optional grammar compilation inside containers
• Support multiple grammar options (none, PHP, all) to cover common use cases without requiring full recompilation
• Add test infrastructure with scenario-based validation, ensuring compatibility with Ubuntu LTS (GLIBC 2.35+) base images
• Extend test-local.sh with --base-image flag to support per-feature base image overrides during local testing

Changes

Feature: tree-sitter

• src/tree-sitter/devcontainer-feature.json: Feature metadata with version and grammars options (none/php/all)
• src/tree-sitter/install.sh: Installation script that downloads the tree-sitter CLI binary and conditionally compiles requested grammars

Tests: tree-sitter

• test/tree-sitter/test.sh: Base test suite verifying CLI is installed and functional
• test/tree-sitter/scenarios.json: Scenario configuration defining three test variants
• test/tree-sitter/base-image.txt: Pins tests to mcr.microsoft.com/devcontainers/base:ubuntu to ensure GLIBC compatibility with tree-sitter binaries
• test/tree-sitter/install_tree_sitter_latest.sh: Scenario script for CLI-only installation
• test/tree-sitter/install_tree_sitter_with_php.sh: Scenario script validating PHP grammar compilation
• test/tree-sitter/install_tree_sitter_with_all_grammars.sh: Scenario script validating all grammars compilation

Test Infrastructure

• test-local.sh: Add --base-image flag support to allow overriding the base image at invocation time, falling back to per-feature base-image.txt configuration

Test plan

• Run ./test-local.sh tree-sitter and verify all three scenarios pass
• Run ./test-local.sh --base-image mcr.microsoft.com/devcontainers/base:ubuntu tree-sitter and confirm CLI flag override works
• Verify tree-sitter --version succeeds inside a container built with the feature
• Verify PHP grammar .so file is present when grammars=php option is set

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