Deprecate Unicode#normalize and Chars#normalize.

[frodsan]

The default normalization used by Unicode and String are different. Unicode#normalize uses :nkfc while String#unicode_normalize defaults to :nfc(ref).

This is why I added the form to the deprecation message so if the method is called without parameters it recommends the correct normalization form for String#unicode_normalize:

>> ActiveSupport::Multibyte::Unicode.default_normalization_form
# => :kc
>> "".mb_chars.normalize
DEPRECATION WARNING: ActiveSupport::Multibyte::Chars#normalize is deprecated 
and will be removed from Rails 6.1. Use #unicode_normalize(:nfkc) instead.

r? @jeremy

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[jeremy]

Excellent. Thank you @frodsan!

[jeremy]

Handling the default case separately is a nice touch that users will appreciate.

[frodsan]

Thanks! 😄