Expose the API via exports instead of editor commands

[TylerLeonhardt]

PR Summary

So my first attempt at doing an external API was totally wrong.

Turns out, extensions can export things that other extensions can use... this is how the Test Explorer extension works:
https://github.com/hbenl/vscode-test-explorer/blob/82e7d339c5ef25c2e02ca5be8f63f46eadd07e8f/src/main.ts#L116-L123

Anyway, not much changes here besides how you access the API... simply:

const powershellExtension = vscode.extensions.getExtension<IPowerShellExtensionClient>("ms-vscode.PowerShell-Preview");
const powerShellExtensionClient = powershellExtension!.exports as IPowerShellExtensionClient;

Then you're good to go:

const sessionId = powerShellExtensionClient.registerExternalExtension("ms-vscode.powershell-test-explorer");
const versionDetails = powerShellExtensionClient.getPowerShellVersionDetails(sessionId);

I mentioned as a TODO in code that this should be wrapped in a simple npm package that hides the management of sessionId and checks for both PowerShell and PowerShell Preview extensions.

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