Add ui_macros crate & DerivePathStr derive macro (#17811)

This PR adds the `ui_macros` crate to allow building supporting macros
for the `ui` crate.

Additionally, it implements the `DerivePathStr` derive macro and the
`path_str` attribute macro. These macros work together to generate a
`path` method for enum variants, which is useful for creating
standardized string representations of enum variants.

The `DerivePathStr` macro provides the following functionality:
- Generates a `path` method for each enum variant.
- Allows specifying a prefix (required) and suffix (optional) for all
paths.
- Supports `strum` attributes for case conversion (e.g., snake_case,
lowercase).

Usage example:

```rust
#[derive(DerivePathStr)]
#[path_str(prefix = "my_prefix", suffix = ".txt")]
#[strum(serialize_all = "snake_case")]
enum MyEnum {
    VariantOne,
    VariantTwo,
}

// Generated paths:
// MyEnum::VariantOne.path() -> "my_prefix/variant_one.txt"
// MyEnum::VariantTwo.path() -> "my_prefix/variant_two.txt"
```

In a later PR this will be used to automate the creation of icon & image
paths in the `ui` crate.

This gives the following benefits:

1. Ensures standard naming of assets as paths are not manually
specified.
2. Makes adding new enum variants less tedious and error-prone.
3. Quickly catches missing or incorrect paths during compilation.
3. Adds a building block towards being able to lint for unused assets in
the future.

Release Notes:

- N/A

crates/ui_macros/src/ui_macros.rs

@@ -0,0 +1,53 @@
+mod derive_path_str;
+
+use proc_macro::TokenStream;
+
+/// Derives the `path` method for an enum.
+///
+/// This macro generates a `path` method for each variant of the enum, which returns a string
+/// representation of the enum variant's path. The path is constructed using a prefix and
+/// optionally a suffix, which are specified using attributes.
+///
+/// # Attributes
+///
+/// - `#[path_str(prefix = "...")]`: Required. Specifies the prefix for all paths.
+/// - `#[path_str(suffix = "...")]`: Optional. Specifies a suffix for all paths.
+/// - `#[strum(serialize_all = "...")]`: Optional. Specifies the case conversion for variant names.
+///
+/// # Example
+///
+/// ```
+/// use strum::EnumString;
+/// use ui_macros::{path_str, DerivePathStr};
+///
+/// #[derive(EnumString, DerivePathStr)]
+/// #[path_str(prefix = "my_prefix", suffix = ".txt")]
+/// #[strum(serialize_all = "snake_case")]
+/// enum MyEnum {
+///     VariantOne,
+///     VariantTwo,
+/// }
+///
+/// // These assertions would work if we could instantiate the enum
+/// // assert_eq!(MyEnum::VariantOne.path(), "my_prefix/variant_one.txt");
+/// // assert_eq!(MyEnum::VariantTwo.path(), "my_prefix/variant_two.txt");
+/// ```
+///
+/// # Panics
+///
+/// This macro will panic if used on anything other than an enum.
+#[proc_macro_derive(DerivePathStr, attributes(path_str))]
+pub fn derive_path_str(input: TokenStream) -> TokenStream {
+    derive_path_str::derive_path_str(input)
+}
+
+/// A marker attribute for use with `DerivePathStr`.
+///
+/// This attribute is used to specify the prefix and suffix for the `path` method
+/// generated by `DerivePathStr`. It doesn't modify the input and is only used as a
+/// marker for the derive macro.
+#[proc_macro_attribute]
+pub fn path_str(_args: TokenStream, input: TokenStream) -> TokenStream {
+    // This attribute doesn't modify the input, it's just a marker
+    input
+}