Add a new Vim script function `showdefinition()` that allows Vim script
to call back to macOS's data lookup feature and show the definition /
URL preview / etc for any text, at a designated row/col position. If the
row/col are not provided this function will just show it at the cursor.
Also, add a new autoload/macvim.vim for utility functions to call
showdefinition() for selected texts and the word under cursor. Make a
new right-click popup menu "Look Up" call that when there are selected
texts in visual mode to make the lookup functionality easier to access
for users without a trackpad (since Ctrl-Cmd-D is a little obscure and
unwieldy to use). For the utility functions, it was a little hard to
determine how to get the text under visual selection without yanking (we
don't want to pollute the register here), and just implemented a
function to take care of all the edge cases including visual/block/line
modes and selection=exclusive. It could be useful in other situations.
As a side refactor, change the message handler in MacVim from if/else to
switch case. In optimized builds, they both essentially optimize to the
same thing, but for debug builds, the if/else statements have to step
through one by one, and switch case just makes more sense for a giant
message ID lookup like this.
Part of Epic #1311
