Naming interfaces and types

[pwalsh]

It is not idiomatic TypeScript to prefix interfaces with I (See here and here, an in general, any of the types from @types), and so on creation of this codebase, I didn't carry over the usage of I prefixed interfaces in general from the internal codebase it is based on.

As documented, I did retain I prefixes for a special meaning - as a way to indicate interfaces that can be overridden by plugins.

Some recent additions used the I prefix for non-pluggable interfaces. Let's discuss, so that the codebase remains consistent.

[pwalsh]

We are going to stick with using I for interfaces that can be overridden by plugins.