existsSync should never throw ENAMETOOLONG (#10446)

* existsSync should never throw ENAMETOOLONG

* undo PathLike change and use better guard

* Apply formatting changes

* await promise

Co-authored-by: Jarred Sumner <jarred@jarredsumner.com>

---------

Co-authored-by: Jarred Sumner <jarred@jarredsumner.com>
Co-authored-by: Jarred-Sumner <Jarred-Sumner@users.noreply.github.com>

src/string.zig

@@ -1305,6 +1305,10 @@ pub const SliceWithUnderlyingString = struct {
         return this.utf8.slice();
     }
 
+    pub fn sliceZ(this: SliceWithUnderlyingString) [:0]const u8 {
+        return this.utf8.sliceZ();
+    }
+
     pub fn format(self: SliceWithUnderlyingString, comptime fmt: []const u8, opts: std.fmt.FormatOptions, writer: anytype) !void {
         if (self.utf8.len == 0) {
             try self.underlying.format(fmt, opts, writer);