`AddImportsVisitor`: Add the ability to import in an `if TYPE_CHECKING` block

When using libcst to add types, we often want to add imports in a conditional `if TYPE_CHECKING` block, to avoid circular imports or improve performance.

It would be great to have an extra `if_type_checking: bool` parameter to `AddImportsVisitor`, that would add the import in the block (and import `TYPE_CHECKING` from `typing` if it is not already the case).

If you think this is a good idea, I may work on this.