Add new spectral command line command to lint with oas ruleset

[DavidBiesack]

User story

As a Spectral user, I can lint an OpenAPI document with minimum fuss, so that I can benefit from Spectral without extra configuration.

Is your feature request related to a problem?
It is very tedious and a poor developer experience to require a -r / -- ruleset CLI option when I'm linting an OpenAPI document. (options should be just that - optional).

Worse, this also requires creating a ruleset file somewhere and referencing it.

Follow the "convention over customization" principle to make Spectral easier to use.

Note: the help page is wrong since it omits the -r option:

spectral lint petstore.yaml

this won't work unless one has created a .spectral.yaml file in the current directory

Describe the solution you'd like

One option is to do a simple content check on the source file if a ruleset is not passed on the command line or is not present at ./.spectral.yaml. I.e. if the file is valid JSON or YAML, and the file has a top-level "openapi" (or even "swagger") value, then default to the oas ruleset ( spectral:oas ).

Another option is to add a second command line command name such as spectral-oas which defaults to the oas ruleset. One can always override the default with a command line option SImilarly, define spectral-asyncapi and spectral-arazzo commands to use just those rulesets.

A third option is to default to a ruleset as described here:

extends: ["spectral:oas", "spectral:asyncapi", "spectral:arazzo"]

whenever the -r option is omitted and there is no ./spectral.yaml file.

A final option is to look in $HOME/.spectral.yaml (or $HOME/config/.spectral.yaml) so spectral can work in arbitrary locations that contain just an openapi document.

Although Spectral can load a ./.spectral.yaml file (if it exists), that file seldom exists. In my experience, most code repos or OAS document downloads do not include a .spectral.yaml along with the OpenAPI source (pick your favorite developer portal that allows OAS downloads).