neural networks – How to train custom named entity recognition?

Version 12 has basic NER support for some entities, but how does one recognize a custom entity?

For example, I want to parse text describing products, and parse out three entities: prices, size, and color, as custom entities. Using TextCases and TextContents works nicely for price and color:

TextCases["A red tshirt costs $5 and is medium", {"Color", 

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But there is no way to parse entities that are not listed in guide/TextContentTypes, like sizes:

TextCases["large women's leather jacket", {"Size"}]

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And there is no way to add additional synonyms or spellings to an entity:

TextCases[# <> " feather", "Color"] & /@ {"violet", "lilac", 
  "lavender", "royal", "purpled", "plum", "grape", "maroon", 
  "magenta", "purplish"}

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I want to extend the built-in NER model with custom training data, i.e. substring labels:

newTrainingData = <|"Who is Nishanth?"-> {8, 16, "Name"},
"Who is Kamal Khumar?", {8, 20, "Name"},
"I like London and Berlin.", {{8,14, "City"}, {19,25,"City"}

If this is not supported in 12.2, perhaps there is a 3rd party paclet, resource function, or some NN repo entry to extend? or maybe it’s coming in 12.3?