Speed up BufferApiView.getLine and remove BufferLineApiView

[PerBothner]

The IBufferApi in the "public API" has a getLine method which is implemented by (on each call) creating a new BufferLineApiView object as a proxy for a BufferLine. This is quite expensive. Other parts of the API just coerce the internal interface to the public API interface. We can't coerce BufferLine to a IBufferLineApi because BufferLine doesn't implement getCell. However, it is trivial to add getCell, so I did.

[PerBothner]

This is part of the large PR #6160.

[jerch]

Getting rid of costly abstractions is a good thing. ❤️

One minor nag: Please add a comment in the bufferline source, that the internal API is directly exposed to public as a warning. I am not sure, if the public API paths are fully tested and the least we want is surprising sideeffects on public API from internal changes.

[PerBothner]

Not sure I'd say "the internal API is directly exposed to public" - you still have to do a cast - and I suspect you have to go via unknown: pline as unknown as BufferLine.

As opposed to the old code: pline._line using the _line pointer to the internal BufferLine object.

It seems weird adding a comment in BufferLine about the public API, since in general the internal APIs don't mention the public APIs.