Add support for `format: "regex"` in strings

[connor4312]

Required for microsoft/vscode#71145

Spec: https://json-schema.org/draft-07/draft-handrews-json-schema-validation-01#rfc.section.7.3.8

[jeremyfiel]

format is not required for validation per the spec

it's informational only, or as JSON Schema call it, an annotation

https://json-schema.org/draft-07/draft-handrews-json-schema-validation-01#rfc.section.3.2

[connor4312]

The JSON spec does not describe them as informational only as far as I know, it merely says (in section 3.2 and 7.2) that asserting on them is optional and that implementations should not fail when encountering a format they do not recognize (in section 7.1).

This is a request to implement that optional part of the specification in the json language service.

[jeremyfiel]

3.3. Annotations
In addition to assertions, this specification provides a small vocabulary of metadata keywords that can be used to annotate the JSON instance with useful information

[connor4312]

Saying that format provides useful information does not mean it is solely informational 🙂