selector/runes.go: isAlpha not robust

[3052]

https://github.com/blend/go-sdk/blob/95d703ff2556c7bfe60d6c8f9c3ee212a8d8f45d/selector/runes.go#L35-L37

for example, this returns true:

fmt.Println(isAlpha(0x200E))

https://wikipedia.org/wiki/Left-to-right_mark