Support dump diff in --check mode (#32)

Originally, when a difference existed between the original and formatted
file under --check mode, the resulting diff was not shown to the user.

This patch enables dumping the diff to the console, providing clear
visibility into what changes were made by the formatting process.

Before:
$ dtsfmt --check minimal.dts
minimal.dts

Errors found while formatting!

After:
$ dtsfmt --check minimal.dts
minimal.dts
-
-
+
+
-
-
-
-            clock-frequency = <5000000>; /* the baudrate divisor is ignored */
-        };
-
-reboot {    value = <0x7777>;    offset = <0x00>;    regmap = <0x04>;    compatible = "syscon-reboot";   };
+            clock-frequency = <5000000>;
+            /* the baudrate divisor is ignored */
+        };
+        reboot {
+            value = <0x7777>;
+            offset = <0x00>;
+            regmap = <0x04>;
+            compatible = "syscon-reboot";
+        };
-

Errors found while formatting!

Author: ChinYikMing

src/emitter/files.rs

@@ -1,5 +1,8 @@
 use std::fs;
 
+use console::Style;
+use similar::{ChangeTag, TextDiff};
+
 use super::*;
 
 #[derive(Debug, Default)]
@@ -14,10 +17,32 @@ impl FilesEmitter {
 impl Emitter for FilesEmitter {
     fn emit_check(
         &mut self,
-        FormattedFile { filename, .. }: FormattedFile<'_>,
+        FormattedFile {
+            filename,
+            original_text,
+            formatted_text,
+        }: FormattedFile<'_>,
     ) -> Result<EmitterResult, io::Error> {
         println!("{}", filename.display());
 
+        let diff = TextDiff::from_lines(original_text, formatted_text);
+
+        for op in diff.ops() {
+            for change in diff.iter_changes(op) {
+                let (sign, style) = match change.tag() {
+                    ChangeTag::Delete => ("-", Style::new().red()),
+                    ChangeTag::Insert => ("+", Style::new().green()),
+                    ChangeTag::Equal => continue,
+                };
+
+                print!(
+                    "{}{}",
+                    style.apply_to(sign).bold(),
+                    style.apply_to(change),
+                );
+            }
+        }
+
         Ok(EmitterResult::default())
     }