Add support for the `a` and `L` regex flags

[erezsh]

Lark currently only supports the flags imslux, however Python supports two additional ones, which perhaps some people will find useful.

While users can still use inline flags, it might be nice for Lark to support it as part of the syntax, and it should be pretty easy.

https://docs.python.org/3/library/re.html#flags

[MegaIng]

I recall there being a reason to not support these flags. I think they can't be set locally within a group, meaning we can't actually support them?

[erezsh]

I'm not sure. But according to the docs, (?a) and (?L) are supported. Maybe the limitation changed for newer versions?

[MegaIng]

Aha, yes, this changed in 3.7 and I didn't notice. So we can change this.