Fix incorrect format string causing invalid export files

Zig requires alignment to be specified when specifying a fill character,
as otherwise digits specified after ':' are interpreted as part of the
field width.

The missing alignment specifier caused character codes < 0x10 to be
serialized incorrectly, producing an export file ncdu could not import.

For example, a character with code 1 would be serialized as '\u00 1'
instead of '\u0001'.

A directory of test files can be generated using:

    mkdir test_files; i=1; while [ $i -le 255 ]; do c="$(printf "$(printf "\\\\x%02xZ" "$i")")"; c="${c%Z}"; touch "test_files/$c"; i=$((i+1)); done

src/scan.zig

@@ -96,7 +96,7 @@ fn writeJsonString(wr: anytype, s: []const u8) !void {
             0xC  => try wr.writeAll("\\f"),
             '\\' => try wr.writeAll("\\\\"),
             '"'  => try wr.writeAll("\\\""),
-            0...7, 0xB, 0xE...0x1F, 127 => try wr.print("\\u00{x:02}", .{ch}),
+            0...7, 0xB, 0xE...0x1F, 127 => try wr.print("\\u00{x:0>2}", .{ch}),
             else => try wr.writeByte(ch)
         }
     }