Consider allowing ifvalue.VALUE to be a regexp

[duglin]

so people don't need to repeat siblingattributes in multiple ifvalues just to support more than one VALUE adding the same set of attributes.

We'd need to decide what to do about cases where multiple ifValues match the attribute we're checking. Some options:
1 - generate a runtime error - but the user can't fix it until less they have write access - or possibly might need admin access to change the model
2 - try to merge all attributes, and conflicts generate a runtime error
3 - try to merge all attributes, and conflicts result in undefined behavior (e.g. possibly "last wins")

[duglin]

Proposal:

• defer to post-v1
• then support it but if there's more than one match then it's "undefined" - possibly, first wins but it's a map so it's unordered.

[duglin]

Make '^' a reserved "leading" character - reserved for future use.

Options for later:

{
  "foo": { .... attr...},
  "^.*space$": { ... },   ends with 'space'
  "^.*space/": { ... }  has 'space/'
  "^space": { ... }  - starts with
}

move support for regexp to post-v1

[github-actions]

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