More permissive quoting in commands

[LightAndLight]

The following should be allowed:

# test.ipso

main : IO ()
main =
  comp
    bind mondayDate <- cmd.read `date -I --date="this monday"`

It currently results in a parse error:

test.ipso:6:49: error: expected one of: '$', '${', '`', command fragment, space
  |
6 |     bind mondayDate <- cmd.read `date -I --date="this monday"`
  |                                                 ^

Double quotes aren't accepted in that position. To be more in line with POSIX-style shells, quotes should be accepted here.

To work around this, quote the entire argument:

    bind mondayDate <- cmd.read `date -I "--date=this monday"`