Add a postAction for running other templates

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

Currently there's issue #3208 open to address several pain points, among them is the pain of having to maintain the same .editorconfig file in multiple templates with a suggested solution of adding some common template features behind some parameters. This is a very specific solution that doesn't address the wider problem of having similar needs for file reuse.

### Describe the solution you'd like.

Your pain with .editorconfig is not unique to it. Internally we have a few different files that are necessary to exist at the root level in every repository. Specifically for this issue, separate from the other features discussed, I think there should be a postAction for running other templates. Policy to always include a global.json file? Add a post action to run the globaljson template. Policy to always include a build script? Add a post action to run a buildscript template. Policy to always include a Directory.Build.props? Add a post action to run a directorybuildprops template. Etc.

There's minor concern for specifying a template that's not installed so you could consider some way of specifying template dependencies, but honestly just producing an error when the template doesn't exist is good enough.

### Additional context

_No response_