Improve compiler diagnostic when HandlerFn is not implemented

For example, if I add a `u64` parameter to a handler, the compiler now
reports this:

```
error[E0277]: Invalid command handler
  --> sable_ircd/src/command/handlers/who.rs:5:1
   |
5  | #[command_handler("WHO")]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^ `for<'a, 'b, 'c, 'd, 'e> fn(&'a server::ClientServer, &'b sable_network::prelude::Network, &'c (dyn command_response::CommandResponse + 'c), source_types::UserSource<'d>, &'e str, u64) -> Result<(), command::error::CommandError> {handle_who}` is not a valid command handler
   |
   = help: the trait `handler::HandlerFn<'_, _, _>` is not implemented for fn item `for<'a, 'b, 'c, 'd, 'e> fn(&'a server::ClientServer, &'b sable_network::prelude::Network, &'c (dyn command_response::CommandResponse + 'c), source_types::UserSource<'d>, &'e str, u64) -> Result<(), command::error::CommandError> {handle_who}`
   = note: All parameter types must implement `AmbientArgument` or `PositionalArgument`
   = note: Return type must be `CommandResult`
note: required by a bound in `call_handler`
  --> sable_ircd/src/command/plumbing/mod.rs:47:20
   |
45 | pub(crate) fn call_handler<'a, Amb, Pos>(
   |               ------------ required by a bound in this function
46 |     ctx: &'a dyn Command,
47 |     handler: &impl HandlerFn<'a, Amb, Pos>,
   |                    ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `call_handler`
   = note: this error originates in the attribute macro `command_handler` (in Nightly builds, run with -Z macro-backtrace for more info)
```

instead of this:

```
error[E0277]: the trait bound `for<'a, 'b, 'c, 'd, 'e> fn(&'a server::ClientServer, &'b sable_network::prelude::Network, &'c (dyn command_response::CommandResponse + 'c), source_types::UserSource<'d>, &'e str, u64) -> Result<(), command::error::CommandError> {handle_who}: handler::HandlerFn<'_, _, _>` is not satisfied
  --> sable_ircd/src/command/handlers/who.rs:5:1
   |
5  | #[command_handler("WHO")]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `handler::HandlerFn<'_, _, _>` is not implemented for fn item `for<'a, 'b, 'c, 'd, 'e> fn(&'a server::ClientServer, &'b sable_network::prelude::Network, &'c (dyn command_response::CommandResponse + 'c), source_types::UserSource<'d>, &'e str, u64) -> Result<(), command::error::CommandError> {handle_who}`
   |
note: required by a bound in `call_handler`
  --> sable_ircd/src/command/plumbing/mod.rs:47:20
   |
45 | pub(crate) fn call_handler<'a, Amb, Pos>(
   |               ------------ required by a bound in this function
46 |     ctx: &'a dyn Command,
47 |     handler: &impl HandlerFn<'a, Amb, Pos>,
   |                    ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `call_handler`
   = note: this error originates in the attribute macro `command_handler` (in Nightly builds, run with -Z macro-backtrace for more info)
```

This uses the `diagnostic::on_unimplemented` attribute stabilized by
[Rust 1.78](https://blog.rust-lang.org/2024/05/02/Rust-1.78.0.html)

sable_ircd/src/command/plumbing/handler.rs

@@ -1,9 +1,23 @@
 use super::*;
 
+// TODO: once rustc implements support for it, use `if(not(Self::Output=CommandResult))`
+// to pick the right note to display.
+#[diagnostic::on_unimplemented(
+    message = "Invalid command handler",
+    label = "`{Self}` is not a valid command handler",
+    note = "All parameter types must implement `AmbientArgument` or `PositionalArgument`",
+    note = "Return type must be `CommandResult`"
+)]
 pub trait HandlerFn<'ctx, Ambient, Positional> {
     fn call(&self, ctx: &'ctx dyn Command, args: ArgListIter<'ctx>) -> CommandResult;
 }
 
+#[diagnostic::on_unimplemented(
+    message = "Invalid command handler",
+    label = "`{Self}` is not a valid command handler",
+    note = "All parameter types must implement `AmbientArgument` or `PositionalArgument`",
+    note = "Return type must be `CommandResult`"
+)]
 pub trait AsyncHandlerFn<'ctx, Ambient, Positional>: Send + Sync {
     fn call(
         &'ctx self,