@page should look at malformed directives and attempt to classify a document as a page.

[NTaylorMullen]

With the new Razor error story we transform malformed @page directives into MalformedDirectiveIRNodes which in turn result in pages not being classified as Razor pages. This impacts the way Mvc routes to said pages; we should be smarter and also inspect malformed directives to determine our "pageness"

[rynowak]

@NTaylorMullen - done?

[NTaylorMullen]

@rynowak nope, waiting for your review on the PR.