Add a public API to enable add-on extensions

Summary of the new feature

As a writer of a PowerShellEditorServices extension I want to enable users to install a vscode extension containing my module.

I need that extension to be able to:

1. Use the integrated console to install the module from the gallery, or provide an additional PSModulePath
2. Import the PSES extension module on vscode extension activation

I want that extension to be able to:

1. Register new requests and notifications that PSES would redirect to my module

The current install process is as follows:

1. Install the module manually from the gallery
2. Add an Import-Module statement to your profile
3. Manually add keybinds to keybinds.json (using syntax you can only determine from reading vscode-powershell's source or a posted example)

That's a lot to ask of folks just to try out a module.

Proposed technical implementation details (optional)

The getExtension function allows vscode extensions to acquire a public API from another extension. This API would need to be surfaced by vscode-powershell internally, and optionally (though ideally) a typescript type definition would also be published to npm.

The bare minimum needed in this API would be an exported function that just imports a module from a path. Something like:

tryImportPowerShellModule(path: string) => bool

Edit: Actually a simple import function would be pretty inconsistent since it would be lost every time the console was restarted/version changed. Maybe something like:

registerPSESExtensionModule(path: string) => void;

Where vscode-powershell internally tracks registered modules and reimports them at startup.

The issue identifies VS Code's getExtension API as the integration entry point and suggests a public TypeScript API, with optional npm type definitions. Start by tracing how vscode-powershell could surface that API and retain registered modules across console restarts; done means extensions can register or import their module and optionally register redirected requests and notifications.