Comment detecting problem in the command `source`

As stated in :

> It is not possible to add a comment to a shell command ":!cmd" or to the ":map" command and a few others (mainly commands that expect expressions), that see the '"' as part of their argument:"

But in handleExSourceCommand, this is not considered:

https://github.com/hluk/FakeVim/blob/3333aa19a3eefe6b2c565da0b3a055b159d71a2b/fakevim/fakevimhandler.cpp#L6591-L6594

This was originally discovered in cpeditor/cpeditor#828, where `nnoremap c :norm ggVG"+` is sourced and becomes `ggVG` without `"+`.

A quick fix is to detect whether the `"` is at the start of a WORD, and maybe also add a special check for `map`. But it will be better if FakeVim can strictly follow how Vim does this.