Put an upper limit on UTF8 file sizes

This is for #7223

* Start the discussion at limiting files to 4MB and under before we refuse to load them
* Display a message if we've reached the limit opening a file
* Implemented in AppShellFIleSystem which is really the only place this could go without rearchitecting a lot of file system work but looking for feedback there too
This is an absolute limit users can't get around, so I don't think we should be too conservative in the name of avoiding potential performance slowdowns.  The goal in #7223 is just to eliminate flat-out freezes and crashes from insanely huge files.  Brackets will happily open a 5 MB file on my machine, so the 4 MB limit seems too low.

The file size reported in #7223 was _2.2 GB_, which may be a tad high to use as the limit :-)  But I tried opening a 68 MB text file in Brackets and it also caused an out of memory crash.  So maybe we should put the limit somewhere in the 10-20 MB range?