Improve EDB next analysis with placeholder step

[Troublor]

EDB engine needs to identify whether a step is a PlaceholderStatement to achieve a precise analysis of what is the next step (when the user presses n to step over).
Originally, EDB's AST analysis module did not recognize the PlaceholderStatement as a step so a heuristic-based approach had to be adopted.

Now, with #21, EDB's AST analysis module has had the ability to identify a PlaceholderStatement step (by uncomment the following lines:

edb/crates/engine/src/analysis/step.rs

Lines 380 to 390 in 141c7e5

	// let src = placeholder_statement.src.into();
	// step!(
	// Placeholder,
	// *placeholder_statement.clone(),
	// src,
	// StepHookLocations { before_step: src.start, after_step: vec![src.next_loc()] },
	// StepKind::PlaceholderStatement
	// );
	// // end the placeholder statement step early
	// self.exit_current_statement_step(statement)?;

It is necessary to refactor and improve the next step analysis with the new PlaceholderStatement step.