Feature request: Check other PRs Project modifications before opening a PR

[rikhuijzer]

It is often the case that a PR is already opened to update some compat entry, but it is blocked for some reason or needs more work. It would avoid lots of confusion if CompatHelper would check the open PRs for modifications to the Project.toml.

For example, if there is already a PR opened by someone setting HTTP from 0.9 to 1, then CompatHelper shouldn't suggest the same modification