Draft: Add NotImplementedError, reduce reliance on MethodError in Base

[BioTurboNick]

Following on from @Seelengrab 's suggestion in #50196 and my comment, I whipped up an implementation of it, also moving a subset of MethodErrors thrown from placeholder methods to this new NotImplementedError.

The error shares some code with MethodError, except where MethodError indicates a failure of dispatch, NotImplementedError indicates that dispatch succeeded, but either that a more specific implementation is expected and required.

NotImplementedError()
NotImplementedError(msg)
NotImplementedError(f, args)
NotImplementedError(f, args, msg)
NotImplementedError(f, args, interfacetype)
NotImplementedError(f, args, interfacetype, msg)

ERROR: NotImplementedError: no implementation has been provided matching f(::Type1, ::Type2), expected as part of the interface for <type here>. <msg here>

As part of this draft PR, I've started moving instances of Base using MethodError for non-dispatch failures, with the following understanding (a few I was unsure of):

• MethodError: Function exists but method dispatch failed. Developer should check agreement of method signatures, or user should check number and subtype of arguments.
• NotImplementedError: Dispatch succeeded, but the function indicates that it could be but hasn't yet been implemented for these types. Developer needs to implement it or user needs to define a method with types more specific than the one that was called to fix this. (e.g., convert)
• ArgumentError: Dispatch succeeded, but the function expects the call should never work. (e.g., zero(Any))

There will be a lot of documentation to change around this if it's accepted.

[vtjnash]

Implements #7512