Document TypeScript AppHost toolchain support

[sebastienros]

Description

Track documentation for the new Node-compatible TypeScript AppHost toolchain support added for microsoft/aspire#15812.

We should document:

• which TypeScript AppHost toolchains are supported today (npm, Bun, Yarn, pnpm)
• how the CLI detects the active toolchain from package.json metadata and lockfiles
• the local prerequisites and how aspire doctor reports missing tooling
• that the existing TypeScript AppHost scaffolding flow remains unchanged and this does not add runtime-specific templates
• that Deno is out of scope for this pass

Related

• Support alternative JavaScript runtimes (Bun, Deno) for TypeScript AppHosts aspire#15812