Add support to flatpak based apps

[shirshak55]

[pimterry]

Just to join up the issues here: this PR is intended to fix https://github.com/httptoolkit/httptoolkit/issues/301.

I think the current code here won't work yet, as discussed in that issue, because it needs to match the binary which is in a Flatpak directory not in $PATH. I'm open to exploring fixes for this though, it's tricky but I'm sure it's possible to detect these apps somehow, it's just that I don't know much about Flatpak myself.

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