Ignore subdir compat when `[workspace]` is used

[RomeoV]

Julia 1.12 introduced the workspace feature.
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:

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

[compat]
StatsBase = "..."

[workspace]
projects = ["test"]

and the Project.toml file in test

[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.

[mtfishman]

This would be very helpful and would help with circumventing #516.

[RomeoV]

For the time being I'm using my fork at https://github.com/RomeoV/CompatHelper.jl/tree/only-update-existing where I added an only_update_existing flag that only updates compat entries in a project file if they already have some entry. That sidesteps this issue. If it works fine for a while I'll make a PR.