Command: add import/symbol

[WebFreak001]

Front-ends/IDEs using workspace-d could use this for additional features and this could also be used for Pure-D/serve-d#200, Pure-D/serve-d#197, Pure-D/serve-d#201, Pure-D/serve-d#202

It should add an import to the top of the file
It should not add an import if its already imported

If a symbol is specified and there is a selective import already, only add it to the selection. If its a complete import, do nothing

[WebFreak001]

doing this just with Dscanner is probably not a good idea because it doesnt show selective imports or locations where its imported using the imports switch and instead of using the xml representation of the AST I could just directly use libdparse to get the AST