Add route string syntax to Razor pages and Blazor

[JamesNK]

Fixes https://github.com/dotnet/razor-compiler/issues/352

I've hacked this in. There are things todo:

1. Is this the right way to detect whether a Razor document is a Razor Page or Blazor page?
2. I was surprised when I ran the unit tests and nothing failed. It appears the razor compiler unit tests aren't in the solution file? If they're somewhere in the repo then I still need to update them.

[JamesNK]

It works:

🥳 🎈 🚀 🎉

cc @CyrusNajmabadi

[333fred]

It does look like there are some test failures.

[333fred]

Done review pass (commit 2)

[davidwengier]

At least we know CI seems to build the compiler bits on PRs.. I honestly wasn't sure :)

[333fred]

@dotnet/razor-compiler for a second review please.

[333fred]

Thanks @JamesNK!