Remove Record-based constraints on signature members

[dfreeman]

Currently, people who have class MyComponent extends Component<MyArgs> and are adopting Glint are naturally inclined to change that to Component<{ Args: MyArgs }>. Unfortunately, if the MyArgs type is declared as an interface, that change will fail with a confusing error:

Type '{ Args: MyArgs; }' does not satisfy the constraint 'ComponentSignature'.
  Types of property 'Args' are incompatible.
    Type 'MyArgs' is not assignable to type 'Record<string, unknown>'.
      Index signature is missing in type 'MyArgs'.(2344)

The underlying reason for this is that TS will infer an index signature for type aliases, but is unwilling to do so for interfaces because that's more unsafe due to the potential for declaration merging. Needing to migrate interfaces to type declarations is an annoying adoption papercut, particularly given that it's not apparent from the error message that that's a viable fix.

In practice this constraint doesn't provide a whole lot of value over using object instead, particularly relative to the number of folks who've already run into this index signature issue. We may consider some type-aware linting in the future, but for now this should remove some real pain for people working to adopt Glint.

[chriskrycho]

If only TS would actually let us describe these semantics usefully. 😩