[bug] partialMatch does not match if text that would ordinarily be matched is preceded by newlines

`partialMatch` does not match after newlines: 

```
const std = @import("std");
const Regex = @import("zig-regex/src/regex.zig").Regex;

test "partialMatch after newlines" {
    var reg = try Regex.compile(std.testing.allocator, "a");
    defer reg.deinit();
    try std.testing.expect(try reg.partialMatch("\na")); // fails
}
```
