Feat: move find_matches to a library.

src/lib.rs

@@ -0,0 +1,18 @@
+use anyhow::Result;
+
+pub fn find_matches(content: &str, pattern: &str, mut writer: impl std::io::Write) -> Result<()> {
+    for line in content.lines() {
+        if line.contains(pattern) {
+            writeln!(writer, "{}", line)?;
+        }
+    }
+    Ok(())
+}
+
+#[test]
+fn find_a_match() -> Result<(), Box<dyn std::error::Error>> {
+    let mut result = Vec::new();
+    find_matches("lorem ipsum\ndolar sit amet", "lorem", &mut result)?;
+    assert_eq!(result, b"lorem ipsum\n");
+    Ok(())
+}