[WIP] Add more options to CamelCasePlugin

[wirekang]

This PR will add more properties to CamelCasePluginOptions:

property	description
ignoreSurroundingUnderscores?: boolean	Ignore leading/trailing underscores, add as-is after conversion
removeSurroudingUnderscores?: boolean	Remove leading/trailing underscores before conversion
customMapping?: Record<string,string>	Provide custom conversion rule

In case of #1627, I think _id -> _Id conversion is a bug. So What's the correct one? _id or id? When I tested a bunch of snake_case to camelCase implementations, the golden rule was to remove leading underscores(_id -> id). But it would be great if there are more options to control some opinioned behavior, since there is no standard.

Closes #1627

WIP

First of all, I added a test file to test CamelCasePlugin independently.
Since the maintainer might reject this approach, I haven't implemented anything yet and also haven't declared all the test cases.

[vercel]

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[wirekang]

Currently, this test case is passing. I think it should modified like ['_id', 'id']