api: Add Message::new_text() (#6123)

This adds a function to `Message`:

```rust
    pub fn new_text(text: String) -> Self {
        Message {
            viewtype: Viewtype::Text,
            text,
            ..Default::default()
        }
    }
```

I keep expecting that a function like this must exist and being
surprised that it doesn't.

Open question is whether it should be `pub` or `pub(crate)` - I made it
`pub` for now because it may be useful for others and we currently we
aren't thinking about the Rust API that much, anyway, but I can make it
`pub(crate)`, too (then it can't be used in deltachat-jsonrpc and
deltachat-repl).

I replaced some usages of Message::new(Viewtype::Text), but not all yet,
I'm going to do this in a follow-up, which will remove another around 65
LOC.

deltachat-jsonrpc/src/api.rs

@@ -2118,8 +2118,7 @@ impl CommandApi {
     ) -> Result<u32> {
         let ctx = self.get_context(account_id).await?;
 
-        let mut msg = Message::new(Viewtype::Text);
-        msg.set_text(text);
+        let mut msg = Message::new_text(text);
 
         let message_id = deltachat::chat::send_msg(&ctx, ChatId::new(chat_id), &mut msg).await?;
         Ok(message_id.to_u32())