src: fix return type of method in string_search.h

`Vector::forward()` is supposed to return a `bool`.

PR-URL: #37167
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

src/string_search.h

@@ -32,7 +32,7 @@ class Vector {
 
   // Returns true if the Vector is front-to-back, false if back-to-front.
   // In the latter case, v[0] corresponds to the *end* of the memory range.
-  size_t forward() const { return is_forward_; }
+  bool forward() const { return is_forward_; }
 
   // Access individual vector elements - checks bounds in debug mode.
   T& operator[](size_t index) const {