Investigate TagHelper scanning improvements

[333fred]

Today, we need to scan all referenced assemblies to determine whether each assembly has defined any tag helpers. This is pretty inefficient, and pretty wasteful: most assemblies users reference couldn't have defined any tag helpers, since they don't reference Microsoft.AspNetCore.Razor.dll. Further, even among the assemblies that could have, many won't, so we're wasting a large amount of time looking for tag helpers where none exist. To address this, I'd like to propose the following scanning algorithm for .NET 9:

1. Does the assembly reference Microsoft.AspNetCore.Razor.dll?
   1. If it does, is the version >8? If so, look for an attribute on the module, something like Microsoft.AspNetCore.Razor.ContainsTagHelpers. If that attribute is present, the assembly contains tag helpers and should be searched. Otherwise, it does not contain tag helpers and can be skipped.
   2. If it does, and the version is <=8, scan the assembly. We have no information on whether tag helpers are actually present.
2. If it does not, there cannot be any tag helpers. Don't scan the assembly.

This will require putting a new source generator into the ref packages for Microsoft.AspNetCore.Razor so that anyone that defines tag helpers will get a module attribute automatically. This pattern is inspired by how C# does extension method lookups.

[333fred]

An alternative strategy for the 1.1 and 1.2 subbullets would be to have ContainsTagHelpers take a bool: if the attribute is present, we will then always know whether we need to scan. If the attribute is not present, we always need to scan. The main cost here would be slight assembly size bloat for assemblies that do not define any tag helpers.