Fix --ignored-suffix to fall back to first-line detection

When an ignored suffix is also a registered extension (e.g. `.txt` maps to
"Plain Text"), `get_syntax_for_file_extension` matched the raw extension
before attempting the suffix strip. As a result `--ignored-suffix .txt` had
no effect on such files and first-line/shebang detection was never reached,
so a shell script saved as `*.txt` stayed unhighlighted.

Strip ignored suffixes first and detect on the remainder; only fall back to
the raw extension when no ignored suffix applies. When a suffix is stripped
but the remainder has no syntax, return `None` so the caller can use
first-line detection. Default behavior (no `--ignored-suffix`) is unchanged.

See #2745.

tests/examples/regression_tests/issue_2745.txt

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+NAME="${0##*/}"
+for i in {1..3}; do
+    echo "hello $NAME $i"
+done

tests/integration_tests.rs

@@ -4248,3 +4248,37 @@ fn tcl_shebang_detection_expect() {
         .assert()
         .success();
 }
+
+#[test]
+fn ignored_suffix_enables_first_line_detection() {
+    // A shebang shell script saved with a `.txt` extension is Plain Text by
+    // default (the extension wins). With `--ignored-suffix .txt` the suffix is
+    // stripped before detection, so it falls back to the first line and is
+    // highlighted exactly as if its language were forced to bash. See #2745.
+    let fixture = "regression_tests/issue_2745.txt";
+    let common = ["--color=always", "--decorations=never", "--style=plain"];
+
+    let stdout = |args: &[&str]| -> Vec<u8> {
+        let assert = bat()
+            .args(common)
+            .args(args)
+            .arg(fixture)
+            .assert()
+            .success();
+        assert.get_output().stdout.clone()
+    };
+
+    let forced_bash = stdout(&["--language", "bash"]);
+    let with_ignored_suffix = stdout(&["--ignored-suffix", ".txt"]);
+    let default = stdout(&[]);
+
+    // The fixture really is being highlighted (forcing bash is not a no-op).
+    assert!(
+        forced_bash.windows(2).any(|w| w == b"\x1b["),
+        "forced-bash output should contain ANSI color codes"
+    );
+    // With the ignored suffix, detection matches forced bash highlighting...
+    assert_eq!(with_ignored_suffix, forced_bash);
+    // ...while the default (extension wins) stays plain and differs.
+    assert_ne!(default, forced_bash);
+}