std.zon.parse: add parsing of enum literals as string

[dasimmet]

Hi @MasonRemaley ,

great job at std.zon!
to fully parse the new build.zig.zon schema's name field at runtime, we do not know the complete enum types,
so instead we could enable parsing those fields into slices.
For Example this type:

struct {
        name: []const u8,
}

can represent this ZON:

.{ .name = .packagen_name }

[MasonRemaley]

Good idea.

My only suggested change is to rename enum_literals_as_string -> enum_literals_as_strings, other than that LGTM!

[MasonRemaley]

(Appreciate the tests covering @"..." and putting it behind an option btw, I expected to make those suggestions then saw that you already did both haha.)