feat(StringSlice): support cast string to string slice by comma

[zoumo]

support cast string to string slice by comma

a b -> []string{"a", "b"}
a,b -> []string{"a", "b"}
a b,c -> []string{"a", "b,c"}

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[zoumo]

@bep

[bep]

I wish we could add this, but the introduction of comma, while it feels natural, would be breaking.

{"a,b", []string{"a", "b"}, false},

Someone may depend the above test case returning "a,b".