Remove special-casing for group(_)

[Scott-Guest]

When we originally added group(_) to replace unstructured attributes, we treated it as syntactic sugar where [group(foo1,...,fooN)] expands internally into [foo1,...,fooN].

However, on a second look, this expansion is unnecessary - group(_) is only relevant in the Context object used by KILtoKORE which maps tags in syntax blocks to sets of corresponding Productions.

In this PR, we then

• Remove all the ProcessGroupAttributes logic and instead just parse group(_) when we build the Context.
• Remove the special-cased KeyType.UserGroup
• (Opinionated) Remove the mention of group(_) from the "unrecognized attributes" error message.
  • The hint was useful when we first disallowed unstructured attributes. However, now that group(_) and the whitelist are more established, it feels a bit odd to assume any unrecognized attribute is an attempt to create a group.

As a side effect of this change,

• syntax declarations can now only refer to productions by their klabel or group. Previously, any attribute was permitted, and the priority or associativity would apply to everything with that attribute.
• Cell properties will no longer expand <cell group="foo"> </cell>
  • This should be disallowed entirely, but see Check cell properties for restricted attributes #3982
• We no longer error if a group conflicts with a built-in attribute
  • Before, this error was required lest [group(function)] be treated the same as [function]