Enable nonstandard_macro_braces and enforce [] for children! (#17974)

# Objective

-
[`nonstandard_macro_braces`](https://rust-lang.github.io/rust-clippy/master/index.html#nonstandard_macro_braces)
is a Clippy lint that enforces what braces certain known macros are
allowed to use.
  - For instance, requiring `println!()` instead of `println!{}`.
- I started working on this after seeing
TheBevyFlock/bevy_cli#277.

## Solution

- Enable `nonstandard_macro_braces` in the workspace.
- Configure Clippy so it enforces `[]` braces for `children!`.

## Testing

1. Create `examples/clippy_test.rs`.
2. Paste the following code:

```rust
//! Some docs woooooooo

use bevy::prelude::*;

fn main() {
    let _ = children!(Name::new("Foo"));
}
```

3. Run `cargo clippy --example clippy_test`.
4. Ensure the following warning is emitted:

```sh
warning: use of irregular braces for `children!` macro
 --> examples/clippy_test.rs:6:13
  |
6 |     let _ = children!(Name::new("Foo"));
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider writing: `children![Name::new("Foo")]`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#nonstandard_macro_braces
  = note: requested on the command line with `-W clippy::nonstandard-macro-braces`

warning: `bevy` (example "clippy_test") generated 1 warning (run `cargo clippy --fix --example "clippy_test"` to apply 1 suggestion)
```

Author: BD103

Cargo.toml

@@ -46,6 +46,7 @@ undocumented_unsafe_blocks = "warn"
 unwrap_or_default = "warn"
 needless_lifetimes = "allow"
 too_many_arguments = "allow"
+nonstandard_macro_braces = "warn"
 
 ptr_as_ptr = "warn"
 ptr_cast_constness = "warn"
@@ -91,6 +92,7 @@ undocumented_unsafe_blocks = "warn"
 unwrap_or_default = "warn"
 needless_lifetimes = "allow"
 too_many_arguments = "allow"
+nonstandard_macro_braces = "warn"
 
 ptr_as_ptr = "warn"
 ptr_cast_constness = "warn"

benches/Cargo.toml

@@ -50,6 +50,7 @@ undocumented_unsafe_blocks = "warn"
 unwrap_or_default = "warn"
 needless_lifetimes = "allow"
 too_many_arguments = "allow"
+nonstandard_macro_braces = "warn"
 
 ptr_as_ptr = "warn"
 ptr_cast_constness = "warn"

clippy.toml

@@ -43,3 +43,6 @@ disallowed-methods = [
   { path = "f32::atanh", reason = "use bevy_math::ops::atanh instead for libm determinism" },
   { path = "criterion::black_box", reason = "use core::hint::black_box instead" },
 ]
+
+# Require `bevy_ecs::children!` to use `[]` braces, instead of `()` or `{}`.
+standard-macro-braces = [{ name = "children", brace = "[" }]