Provide native Dock icon when running on macOS Big Sur

[machawk1]

Other native apps have an iOS-like rounded rectangle. The Chrome team tried hard, they really did.

[machawk1]

Started an icon in issue-490 branch but not satisfied and the .ico generation procedure (in README) ought to be automated.

[machawk1]

Build script added as build/resize.sh but the PSD basis is unsatisfactory and there ought to be an OS conditional check to only serve the rounded rectangle icon to the OSes that exhibit this visual pattern.

Also, the icon is somewhat ugly. This should be improved before merging, as above.

[machawk1]

The icon is a little cleaner in c2c2b4a.

The logic to conditionally apply this version needs to be implemented in either the build script (not ideal, two versions) or to somehow adapt it at runtime (a la Cyberduck):

• https://github.com/elrumo/macOS_Big_Sur_icons_replacements/issues/502
• https://github.com/iterate-ch/cyberduck/commit/2d04abc30cdaf5d54a7d952e57614590357ed379

[machawk1]

Removed the letters' dropshadow in af15629 and it subjectively looks cleaner. Also, no need for the Big Sur conditional check, this icon is just generally easier to read, even if small print is not the best choice.