[clang-format] CSharp don't allow there not to be a space between `is` and `[`

as `is` is a keyword in C# ensure there is always a space before the `[` regardless of `SpaceBeforeSquareBrackets` setting

Fixes: #61965

https://github.com/llvm/llvm-project/issues/61965

Reviewed By: owenpan, HazardyKnusperkeks

Differential Revision: https://reviews.llvm.org/D148472