Add support for using @helper across views

[kichalla]

Currently we support using @helper in the same view that they are defined...we are seeing users using this across views.

I briefly researched on if ViewComponents would be a replacement for this but the thing here is that ViewComponents are meant to separate html and backing code where as the @helper stuff mixes @html and c# code, which is by design and expected.

[danroth27]

We should discuss if we still need @helper.

[yishaigalatzer]

If we do helpers, we will need to figure out how to do them outside app_code as we don't have the concept of special folders no more, but what made them work in the past is the fact that they where precompiled upfront. So from a brief thought, this looks like a pre compilation thing, except it seems wierd to have a built in feature rely on the current precompilation system.

I don't think we have to have this feature in for RC, but I'd like to have a high-level design in place to ensure we are not missing any core behaviors that will enable such a feature.

CC @davidfowl @lodejard @rynowak

[yishaigalatzer]

We want to setup a design meeting, mostly to understand the scope and if changes are required to the framework/compilation model. The intent is not to implement for RTM, but to be able to build a preview package

[pranavkm]

Based on the design discussion, the plan is to remove support for @helper until we have a better design for using and sharing them.

[alexaku-zz]

Fucking theorists. In practice, without @Helper, I need to add several hundred new files manually to migrate my two websites in MVC 6. Each such file will be a couple of lines of context dependent html-code.