[clang] [clang-format] Add an option to control indentation of `export { ... }` (PR #110381)

[via cfe-commits]

Sirraide wrote:

> @Sirraide can you update it with another patch to address the following?
> 
> * The new option needs a config parse test.
> * The option should be renamed to `IndentExternBlock`. (Other than a few exceptions, which should probably be renamed as well, we use "IndentSomething" instead of "SomethingIndentation".)

Sure

https://github.com/llvm/llvm-project/pull/110381