[Feature] Support using external shim.exe

[chawyehsu]

Feature Request

Is your feature request related to a problem? Please describe.

By allowing to config Scoop to use external shim.exe, i.e. shim.exe which is manually installed, instead of those bundled with Scoop, users may switch to leverage their own shim.exe builds without relying on updates to the Scoop codebase or the need to patch the supporting files manually, which can result conflicts.

I'm experimenting/maintaining my own shim.exe builds for Windows ARM in https://github.com/chawyehsu/kiennq-shim/tree/zig-build (upstream: kiennq/scoop-better-shimexe#10). While now it's not merged and may not be ready for a GA release, it's good for a try. It will be much easier to try such builds by installing via a manifest rather than replacing bundled shim.exe files if use of external shim.exe is supported.

Describe the solution you'd like

a rough way was drafted in #6208 to support:

scoop install myshim-pkg
scoop config shim 'ext:myshim-pkg'

Describe alternatives you've considered

n/a