Prior to this change, Signal-Desktop was using a rather ancient version
of `mkdirp`, which was meant for Node.js versions without native support
for this kind of functionality. Starting with Node v10, `fs.mkdir()` can
perform recursive directory creation.
Since Signal-Desktop doesn't utilize any advantages of using such an old
version of `mkdirp` [1] (let alone any version of `mkdirp`), we can
replace it with the native methods of `fs` (or `fs/promises`).
This transition slightly reduces the amount of packages needed to be
downloaded and included in the final app since it now relies on the
built-in API of Node.js.
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