Casing function

[P0lip]

User story.
As a ruleset maintainer, I would like to have a built-in casing function I could make use of in my own rulesets.
For the time being, in order to enforce casing, you need to use pattern function and come up with a regular expression.

Describe the solution you'd like
We could make enforcing naming convention in Spectral easier by providing casing function with pre-defined set of regular expressions, such as:

• pascal case
• camel case
• kebab case
• snake case
• etc.

casing function could also take a custom regular expression, same as pattern does.

Here is an example rule definition:

given: $.paths[*].[*].[*].operationId # I know it looks weird, it's not important though
then:
      function: casing
      functionOptions:
        case: pascal

Additional context
It's a complementary functionality to #405

[jerzyn]

take into account the difference of allowed characters in paths, JSON Schema properties, OAS params, query parameters, etc. (for one case convention)

[philsturgeon]

The goal here is to make the core functions, then let #405 create the rules? I'm game for that, let's get this one done sooner rather than later. It's a major use case.