MacVim does not properly copy runtime files to the app bundle

[ychin]

Currently MacVim does not properly install the Vim runtime files to the app bundle. It simply grabs the runtime folder and copy it over but that folder contains files that aren't for deploying to the target runtime folder. In regular Vim installation, Vim's make install step is in charge of the logic to only copy the necessary files over to runtime and also copy some others to other locations like man pages, and the testdir/ files in syntax and indent are not installed anywhere.

We should fix the script to simply use make install instead (and do whatever changes we need if we don't want certain files). This prevents copying over unnecessary files.

Note: #1418 fixed part of this by manually cleaning up some files (mostly the testdir ones) but there are quite a few others, and we should not manually clean them up like this.

One design decision is how to handle things like man pages and so on. Currently MacVim does not install any of that even for say Homebrew builds. We could expose an option to explicit install it. This is similar to #1404 which proposes exposing a functionality to install the mvim CLI tool to PATH.