Ignore subdir compat when `[workspace]` is used

Julia 1.12 introduced the [`workspace` feature](https://docs.julialang.org/en/v1/manual/code-loading/#workspaces).
It would be nice if CompatHelper could consider that and only suggest compat entries for subdirectories that have not already been specified in the "super" Project.toml file.

For instance, consider a package `MyPkg` with Project.toml:

```toml
name = "MyPkg"
[deps]
StatsBase ="..."

[compat]
StatsBase = "..."

[workspace]
projects = ["test"]
```

and the Project.toml file in `test`
```toml
[deps]
MyPkg = "..."
StatsBase = "..."
TestDep = "..."
```

Now notice that the first Project.toml file specifies compat bounds on `StatsBase`. Thus, we don't need to add another compat entry to the `test` Project.toml. However, we may be interested in compat bumps for `TestDep`, so we'd like to add `test` to the `subdirs` of `CompatHelper.main`. However, this workflow doesn't seem well supported yet.