[clang] [clang-format]: Add `Custom` to `ShortFunctionStyle`; add new AllowShortFunctionsOnASingleLineOptions for granular setup (PR #134337)

[Owen Pan via cfe-commits]

owenca wrote:

We should stick to the table in https://github.com/llvm/llvm-project/pull/134337#discussion_r2028339995 and deprecate `SFS_InlineOnly`, `SFS_Empty`, and `SFS_Inline`.

https://github.com/llvm/llvm-project/pull/134337