Add support for code formatting in Obj-C header comments

[brycebostwick]

This would only require a change in commentBody() in Clang+SourceKitten.swift (as best as I can tell!).

Two different scenarios that would be good to support:

Individual @c Word Highlighting

And:

Highlighting an entire codeblock:

@code
print("Hello world!")
@endcode

The first should be implementable via regex / wrapping the targeted word in backticks.

The second can be rougly implemented by just replacing @code and @endcode with triple-backticks, though this would result in different results from Xcode in some scenarios (most of which should be considered error scenarios already / even cause warnings in Xcode, essentially in situations where the keywords are in a different order or count than expected).

[brycebostwick]

I have a basic implementation of the above strategy locally & will open a PR in a bit, but definitely happy to talk about the change here, including any other ways of tackling this!

[jpsim]

Added in #632.