Tag Helpers: support ModelExpression IntelliSense

[dougbu]

for example if the user is at | in the following, IntelliSense should prompt with available fields and properties in the model:

  <input type="text" for="|"/>

this amounts to design-time support for the quoted expression used at runtime to create a ModelExpression instance. could be as simple as projecting the user to the | in the following.

  var __unused = CreateModelExpression(model => model.|);

[yishaigalatzer]

This is already done, tooling needs to verify once the tooling feature lights up. @vkichline