Introduce PipeSeparatedFlags<T> marker class

[flobernd]

What?

This PR introduces the FlagsEnum<T> marker class similar to the Stringified<T> concept.

Flags enums are defined as enums with a special case serialization. Values of this enum are represented as string (like for regular enums as well). In addition, individual enum member values can be combined using the pipe separator char (|) like e.g. described here:
https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-simple-query-string-query.html#supported-flags

Why?

At the moment, this specific construct is defined in 2 parts:

1. The enum itself (TEnum) as a regular enum type
2. A type alias defined as FlagsEnum = TEnum | string

Using just a generic union makes it hard to generate correct (de-)serialization code and "hard" for the users to use properties of this type, as they have to manually craft strings when using multiple values (unintuitive, error prone, ...).

In case of C#, this marker would allow me to e.g. generate a native language construct:

[Flags]
public enum MyEnum
{
    None = 0,
    A = 1 << 0,
    B = 1 << 1,
    ...
}

and allows the user to utilize the standard way of working with flags:

var flags = MyEnum.A | MyEnum.B;
if (flags.HasFlag(MyEnum.B))
{
    ...
}

[flobernd]

Does not make a real difference as these values are currently not part of the generated schema, but I adjusted them anyways based on their actual values:

https://lucene.apache.org/core/8_9_0/queryparser/constant-values.html#org.apache.lucene.queryparser.simple.SimpleQueryParser.AND_OPERATOR

[swallez]

Actually you can remove these constants altogether. They're some ancient leftovers from the very first iterations on the TypeScript spec where integer constants were added to the enums.

[swallez]

LGTM. As mentioned in the comment, we can remove the integer constants in the enum.

[swallez]

Actually you can remove these constants altogether. They're some ancient leftovers from the very first iterations on the TypeScript spec where integer constants were added to the enums.