replace shell scripts with node

[Shane98c]

Any windows devs who want to help out will appreciate this!

• Replaced 3 .sh scripts (sync-fixtures, sync-flatbuffers, ensure-flatc) with cross-platform Node.js (.mjs)
• Consolidated both sync scripts into a single sync.mjs with --if-missing and --branch flags
• Extracted inline shell pipelines (generate:fbs, check:fbs, clean) into proper scripts
• ensure-flatc.mjs now supports Windows (PowerShell download + .exe binary)
• All package.json scripts now follow a consistent node scripts/... pattern

[gemini-code-assist]

Code Review

This pull request migrates several shell scripts to Node.js, improving cross-platform compatibility and maintainability. The review identified several security and robustness concerns, including command injection vulnerabilities in scripts/sync.mjs due to shell interpolation, missing validation for the --branch argument, and potential issues with unquoted paths in git sparse-checkout commands. Additionally, it was suggested to improve the robustness of PowerShell path handling in scripts/ensure-flatc.mjs to prevent failures with special characters.