Add support for @types/xrm@v8

[BetimBeja]

There are still a lot of old on premise implementations working with @types/xrm v8, I happen work on a project like this daily 😀
It would be nice to support those projects too.

[OliverFlint]

I assume there are some functions/apis that are in the current template that are not available in v8?

[BetimBeja]

Yes. There are some api's that are different, and there has been some moving of typings (ex. from Xrm.Page.BooleanAttribute in v8 to Xrm.Attributes.BooleanAttribute in v9)
I must confess that is has been more than a year that I am not engaged in any v8 maintenance myself, and I don't even have access to such environment to test the tool.

[OliverFlint]

ok, I guess we can add and api-version switch, and then I should be able to deal with it all in the template.