First draft: added with-colon statement

[Walter Bright]

This was requested in the newsgroups. Adds the syntax:

```
{
     with (E):
        statement;
        statement;
}
```

which is equivalent to:

```
{
     with (E)
     {
        statement;
        statement;
     }
}
```

and that's all there is to it.

https://github.com/dlang/dmd/pull/20715