terminal: Match trait bounds with terminal input (#31441)

The core change here is the following:

```rust
fn write_to_pty(&self, input: impl Into<Vec<u8>>);

// into
fn write_to_pty(&self, input: impl Into<Cow<'static, [u8]>>);
```

This matches the trait bounds that's used by the Alacritty crate. We are
now allowed to effectively pass `&'static str` instead of always needing
a `String`.

The main benefit comes from making the `to_esc_str` function return a
`Cow<'static, str>` instead of `String`. We save an allocation in the
following instances:

- When the user presses any special key that isn't alphanumerical (in
the terminal)
- When the uses presses any key while a modifier is active (in the
terminal)
- When focusing/un-focusing the terminal
- When completing or undoing a terminal transaction
- When starting a terminal assist

This basically saves us an allocation on **every key** press in the
terminal.

NOTE: This same optimization can be done for **nearly all** keypresses
in the entirety of Zed by changing the signature of the `Keystroke`
struct in gpui. If the Zed team is interested in a PR for it, let me
know.

Release Notes:

- N/A

crates/project/src/terminals.rs

@@ -514,7 +514,7 @@ impl Project {
         terminal_handle: &Entity<Terminal>,
         cx: &mut App,
     ) {
-        terminal_handle.update(cx, |terminal, _| terminal.input(command));
+        terminal_handle.update(cx, |terminal, _| terminal.input(command.into_bytes()));
     }
 
     pub fn local_terminal_handles(&self) -> &Vec<WeakEntity<terminal::Terminal>> {