Combine const and enum handlers

[jdesrosiers]

Currently if we have multiple const or enum keywords apply to the same location we get multiple error messages.

{
  "allOf": [
    { "enum": ["a", "b", "c"] },
    { "enum": ["a", "b"] },
    { "const": "a" }
  ]
}

We'll multiple messages

• Expected one of "a", "b", or "c"
• Expected one of "a" or "b"
• Expected exactly "a"

Most of this is noise for the user and can be reduced to just,

• Expected exactly "a"

For this task, I want to combine the error handlers for enum and const and reduce these keywords to a single const or enum message.