fix(types): declare buffer() deprecated

[dnalborczyk]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• New feature
• Other, please explain: adds a type deprecation to the already existing runtime deprecation.

What changes did you make? (provide an overview)

Which issue (if any) does this pull request address?

Is there anything you'd like reviewers to know?

[LinusU]

Neat 👍

[dnalborczyk]

@LinusU the deprecation message is taken 1:1 from:

node-fetch/src/body.js

Line 152 in 4972e00

Body.prototype.buffer = deprecate(Body.prototype.buffer, 'Please use \'response.arrayBuffer()\' instead of \'response.buffer()\'', 'node-fetch#buffer');

I didn't want to deviate from the above, but I personally think it should rather be something like:

'Body.prototype.buffer()' is deprecated. Please use 'Body.prototype.arrayBuffer()' instead.

as response is the name of the instance, which not everyone would name it response necessarily.

wording similar to:
https://github.com/nodejs/node/blob/d0a898681f8d5a5fcd53fa2ab8e0a3da807791be/lib/internal/bootstrap/node.js#L295

I can do follow-up PR, or push the changes on top of this one.