Summary

• Enables trailing comma support in the microfrontends config parser
• JSONC files commonly use trailing commas (editors/formatters add them), but turbo was rejecting them

Test Plan

• Added trailing-commas.jsonc fixture with comments and trailing commas
• Added tests verifying trailing commas are accepted in both configv1.rs and schema.rs
• Verified fix works end-to-end with a real-world microfrontends config

_{CLOSES TURBO-5129}