Add durable-functions-migration skill with non-.NET examples

Add Copilot skill for migrating Durable Functions apps from legacy
backends (Azure Storage, Netherite, MSSQL) to Durable Task Scheduler.
Includes complete before/after migration examples for Python,
JavaScript/TypeScript, and Java in addition to .NET.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: greenie-msft