[BUG] Operations object should have function channel() instead of channels()

[snailtomatoes]

Describe the bug.

As per spec v3.0.0 Operations are unique and only support a single Channel. Doesn't make much sense to return a Channels object since it should only contain only one Channel.

https://www.asyncapi.com/docs/reference/specification/v3.0.0#operationObject
https://github.com/asyncapi/parser-api/blob/master/docs/api.md

Expected behavior

parser should support operation.channel() and not operation.channels()

Screenshots

How to Reproduce

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👀 Have you checked for similar open issues?

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🏢 Have you read the Contributing Guidelines?

• I have read the Contributing Guidelines

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