Improve navigation of internal methods

[camillobruni]

Very often when I navigate the spec I end up grepping/searching for internal methods in order to figure out who implements a special case. It would be nice if you could either show a quick popup with all the implementations or directly search for it in the TOC.

[bakkot]

We've improved the situation substantially with https://github.com/tc39/ecma262/pull/2271. Things that still need this:

• internal methods like [[GetOwnProperty]]
• the numeric operators like T::unaryMinus