New icon to match Big Sur's HIG

[Ecco]

Describe the bug
MacVim's icon doesn't comply with Big Sur's new guidelines regarding app icons.

Expected behavior
Comply with the guidelines. This way the app will fit better within macOS.

[ychin]

Did you mean the app icon for launching the app? It was recently added in #1054 already. We just haven't pushed out a release to update it yet.

[Ecco]

Oh, yes indeed! Sweet!