Implement NIP-19 bech32-encoded private and public keys

https://github.com/nostr-protocol/nips/blob/master/19.md

sig/nostr/errors/error.rbs

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+module Nostr
+  class Error < StandardError
+  end
+end

sig/nostr/errors/invalid_hrb_error.rbs

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+module Nostr
+  class InvalidHRPError < KeyValidationError
+    def initialize: (String, String) -> void
+  end
+end
+

sig/nostr/errors/invalid_key_format_error.rbs

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+module Nostr
+  class InvalidKeyFormatError < KeyValidationError
+    def initialize: (String) -> void
+  end
+end

sig/nostr/errors/invalid_key_length_error.rbs

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+module Nostr
+  class InvalidKeyLengthError < KeyValidationError
+    def initialize: (String) -> void
+  end
+end

sig/nostr/errors/invalid_key_type_error.rbs

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+module Nostr
+  class InvalidKeyTypeError < KeyValidationError
+    def initialize: (String) -> void
+  end
+end

sig/nostr/errors/key_validation_error.rbs

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+module Nostr
+  class KeyValidationError < Error
+  end
+end