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