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

[Owen Pan via cfe-commits]

owenca 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".)

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