PR checks perf improvements and organization

[timotheeguerin]

Result when from 30-35min pipelines to under 20min

• Use corepack to install pnpm: Faster and respect the pnpm version set in package.json instead of having another place to keep up to date
• dependency cache
• move swagger check to seperate job
• upgrade to new code coverage task
• Don't run core tests in ci
• Move all consitency check to independent github action workflow(Makes it easier to see which one failed immediately without having to open devops and dig into the steps)

[azure-sdk]

All changed packages have been documented.

[azure-sdk]

You can try these changes at https://cadlplayground.z22.web.core.windows.net/typespec-azure/prs/568/

Check the website changes at https://tspwebsitepr.z22.web.core.windows.net/typespec-azure/prs/568/