[Enhancement] dracula.xml for gtksourceview-4 (important for the widely used Meld program)

Hello,

I managed to modify dracula.xml so it can be used for gtksourceview-4, which Meld uses.
Meld is widely used, so this is important.

For a dracula.xml that can be used by gtksourceview-4, these lines need to be removed:
```

dark

```
To install locally:
`cp dracula_gtksourceview4.xml ~/.local/share/gtksourceview-4/styles/dracula.xml`

It works!
Maybe some other modifications are sensible, I don't know much about the format.
Comparing existing color schemes, there are some other changes, too, see:
`diff /usr/share/gtksourceview-4/styles/cobalt.xml /usr/share/gtksourceview-5/styles/cobalt.xml`

Perhaps it would be good to create a branch for gtksourceview-4 in this repo.

It also would be good to create a page https://draculatheme.com/meld, this way people can directly find the Dracula theme for meld.