Add DSL for effect creation and composition

[junkdog]

This PR adds a new DSL feature that allows effects to be created and composed using a string-based syntax, in appearance identical to (a tiny subset of) rust. The DSL makes ad-hoc runtime effect debugging/iteration feasible, plus is a pretty neat serialization format.

New Public API

Core Types

• EffectDsl: Reusable registry and compiler for effect expressions
• DslCompiler: Produces effects from input DSL. Single use.
• EffectExpression: Parsed representation of a DSL expression (typically produced by Effect::to_dsl)
• DslError: Error types for DSL parsing and evaluation
• Shader::to_dsl: (optional) required for serializing effects to DSL strings.
• DslFormat: Helper trait for converting types to DSL representation

Key Functions

// Create and configure any custom DSL compilers
let dsl = EffectDsl::new()
    .register("custom_effect", |args| { ... });

// Create a compiler and bind variables
let effect = dsl.compiler()
    .bind("color", Color::Blue)
    .bind("duration", 1000)
    .compile("fx::fade_to(color, (duration, Linear))");

// Convert effects to DSL expressions
let expr = effect.to_dsl()?;
println!("{}", expr); // Prints formatted DSL code

Features

• String-based syntax for creating and composing effects
• Variable binding for dynamic values
• Type-safe argument parsing and validation
• Pretty-printing of DSL expressions
• Extension point for custom effects

Examples (tachyonfx DSL)

Basic Effect

fx::fade_to(Color::from_u32(0), (500, Linear))

Complex composition

The DSL, while limited in scope, has bindings and parsers for the more frequently used types and methods.

// line comments are supported by the DSL 
/* and block comments */
let layout = Layout::horizontal([Percentage(50), Percentage(50)])
    .spacing(1)
    .vertical_margin(1)
    .horizontal_margin(2);

let cell_filter = CellFilter::Not(Box::new(CellFilter::Layout(layout, 1)));
let content_area = Rect::new(0, 0, 10, 10);

fx::sweep_in(
    Motion::LeftToRight,
    10,
    0,
    Color::from_u32(0x1d2021), 
    EffectTimer::from_ms(1000, QuadOut)
).with_filter(cell_filter).with_area(content_area);

(The DSL above is valid rust code too, in case it wasn't obvious)

Implementation notes

• Uses a recursive descent parser for expressions
• Leverages trait system for type-safe argument handling
• No breaking changes to the effect API
• Performance overhead is only at creation time

todo

• parser-related error reporting
• docs
• add example
• parser: discard comments
• parser: Offset
• simple let assignments
• fix remaining todo:s
• ops on Rect
• ensure all parsers correctly identify vars
• consider simplifying complex parsers with argument() parser
• parser: CellFilter + apply filter, except Fn-based
• parser: RepeatMode + repeat compiler
• parser: Layout
• use compact_str instead of String
• Shader::to_dsl/Shader::to_expr
• parser: Rect
• parser: Margin
• FxArg::Var (ref variables must be registered with the script exec context)
• resolve ambiguity: u32, Duration, EffectTimer
• script exec context (registered serializers, bound variables; rects, colors etc)
• deserialization functionality (read args, auto match against input)
• "compiler"-related error reporting
• save effect instance to String with pretty formatting

the future

• rework parsers; split into tokenization and expr parsing
• improve struct parsing to be agnostic to field declaration order
• wasmify dsl playground
• better type introspection for bound variables; remove need for chained method to return self