feat: Add a vendor directory option to use a custom vendor

[beyondkmp]

Doing this has two benefits:

1. Users can use their own compiled and modified version of Squirrel Windows.
2. Users can perform their own checks during the download to ensure the executables in the vendor directory have not been tampered with.

[felixrieseberg]

@beyondkmp This is looking pretty good - and something that scares us maintainers a lot less than upgrading NuGet or 7-zip directly because letting people customize their vendors comes without any backwarts compat issues. We don't have any specific reason to suspect that either the 7Zip or NuGet upgrade would break anything, but then again, we haven't tested it either - and if they do break things, we'd rather not be on the hook for fixing it 😉 Your change here seems like a great solution!

Thank you!

(Note for others: Check out the changes without whitespace changes)

[continuous-auth]

🎉 This PR is included in version 5.4.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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