Support Subscript builders

[lickel]

I wanted to add conformance to @dynamicMemberLookup in some generated code.
This is not possible as there's no way (that I can tell) to create subscripts.

Specifically, I wanted to create something like subscript<Property>(dynamicMember keyPath: KeyPath<BackingData, Property>) -> Property.

There doesn't seem to be any "escape hatch" for TypeSpec.Builder to just type "raw" Swift code.
As such, I think the only way to solve this would be a dedicated function/builder.

[kdubb]

What's wrong with creating a function named subscript?

[lickel]

There's two problems:

• I need to support both the getter and setter flavors
• the FunctionSpec builder notices it's a keyword and escapes it

[kdubb]

Got it. I tend to forget that subscript is kind of a special property.

That being the case... adding support to PropertySpec should be easy.

[lickel]

Yeah I figured it should be relatively straightforward since it's basically a special property except that it shares the flexibile generics and method signatures of a function.