Expose API for registering callbacks when the PDF is clicked

[JJK96]

Possibility for registering a callback for receiving the piece of text that was clicked.

I'm working on an extension to synchronize this to my source format (similar to synctex).
Such an extension could hook into pdfviewer like this:

	var pdfviewer = vscode.extensions.getExtension('tomoki1207.pdf');
	if (pdfviewer) {
		pdfviewer.activate().then((api) => {
			api.registerOnClickCallback((content: string) => {
				channel.appendLine(content)
			})
		})
	}