[Refactor] Introduce builder pattern for Emitter construction

[sorafujitani]

Summary

Introduce a builder pattern for Emitter construction to improve API ergonomics and support future configuration options.

Background

File: ext/rfmt/src/emitter/mod.rs:29-52

impl Emitter {
    pub fn new(config: Config) -> Self { ... }
    pub fn with_source(config: Config, source: String) -> Self { ... }
}

Problem

Multiple constructors with different parameter combinations:

• Hard to extend without breaking changes
• Unclear which constructor to use
• No compile-time validation of required fields

Proposed Solution

Step 1: Define EmitterBuilder

#[derive(Debug, Default)]
pub struct EmitterBuilder {
    config: Option<Config>,
    source: Option<String>,
}

impl EmitterBuilder {
    pub fn new() -> Self {
        Self::default()
    }

    pub fn config(mut self, config: Config) -> Self {
        self.config = Some(config);
        self
    }

    pub fn source(mut self, source: impl Into<String>) -> Self {
        self.source = Some(source.into());
        self
    }

    pub fn build(self) -> Emitter {
        Emitter {
            config: self.config.unwrap_or_default(),
            source: self.source,
            // ... other fields initialized to defaults
        }
    }
}

Step 2: Add convenience method on Emitter

impl Emitter {
    pub fn builder() -> EmitterBuilder {
        EmitterBuilder::new()
    }

    // Keep existing constructors for backward compatibility
    pub fn new(config: Config) -> Self {
        Self::builder().config(config).build()
    }

    pub fn with_source(config: Config, source: String) -> Self {
        Self::builder().config(config).source(source).build()
    }
}

Usage

// Before
let emitter = Emitter::with_source(config, source);

// After (both work)
let emitter = Emitter::with_source(config, source);  // backward compatible
let emitter = Emitter::builder()
    .config(config)
    .source(source)
    .build();

Benefits

1. Extensible - New options don't require new constructors
2. Discoverable - IDE autocomplete shows all options
3. Backward compatible - Existing code continues to work

Files to Modify

• ext/rfmt/src/emitter/mod.rs - Add EmitterBuilder

Acceptance Criteria

• EmitterBuilder struct implemented
• Emitter::builder() method added
• Existing constructors preserved for backward compatibility
• All existing tests pass
• Documentation added for builder pattern