feat: add $for loop separator options

[adamcavendish]

Summary

• add separator = expr and trailing = bool options to $for
• keep statement $for separators literal while adding true inline $for fragment joins
• support multiline inline separators for union-style continuations, such as TypeScript | members
• document the inline-vs-statement mental model and refresh macro feature examples
• add coverage for inline unions, import tracking, trailing separators, statement separators, compound separators, and Go pointer spacing

Semantics

• statement $for emits statement or block chunks and preserves their rendered newlines
• inline $for acts like a join expression inside the surrounding statement
• trailing = true repeats the same separator after the final non-empty iteration
• empty loops emit no separator or trailing separator

Verification

• cargo test
• just lint
• cargo run --example macro_features
• mdbook build docs
• mdbook test with an isolated CARGO_TARGET_DIR
• git diff --check

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