Catch2::Catch2Main CMake target in v3 preview seems to create lots of work for users

**Describe the bug**

I think that lots of Catch2 v2 users are going to have to make lots of repetitive edits to their CMakeLists.txt files....

I have a dozens of projects that use use Catch2 and ApprovalTests...

Their CMake `target_link_libraries` used to look like this:

```cmake
target_link_libraries(${PROJECT_NAME} PRIVATE ApprovalTests::ApprovalTests Catch2::Catch2)
```

These all use `CATCH_CONFIG_MAIN`, or occasionally `CATCH_CONFIG_RUNNER`...

I'm guessing that the vast majority of Catch2 users will use `CATCH_CONFIG_MAIN`.

In order to work with the Catch2 v3 preview release, I have had to change all of them to this:

```cmake
target_link_libraries(${PROJECT_NAME} PRIVATE ApprovalTests::ApprovalTests Catch2::Catch2)
if (TARGET Catch2::Catch2Main)
# link Catch2::Catch2Main only if it exists (so probably catch2 v3)
target_link_libraries(${PROJECT_NAME} PRIVATE Catch2::Catch2Main)
endif()
```

Am I right in assuming that every project that uses `CATCH_CONFIG_MAIN` will have to add `Catch2::Catch2Main` in v3 - and that if they wish to support v2 and v3 for a period, they will need the `if` block above as well?

**Expected behavior**

That v3 should ideally use the same CMake as v2...

**Idea**

Is it possible to make the CMake target that includes the main() be `Catch2::Catch2` - and to have the library one be something like `Catch2::Catch2WithoutMain`?