Dotty currently uses three syntactically different schemes for extension methods

 1. Single methods with prefix parameters such as `def (x: T).f(y: U)` or `def (x: T) op (y: U)`.
    These methods can be abstract, can override each other, etc, which is crucial for infix ops
    in type classes.
 2. Given instances defining (only) extension methods with syntax `extension ops { ... }`
 3. Collective extension methods with syntax `extension on (x: T) { ... }`

This PR proposes another scheme that unifies (1) and (3) and all but eliminates the need for (2).

The simplification is achieved by redefining many fundamentals of extension methods. In a sense,
the price we pay for user-facing simplifications is a more complex implementation scheme. But that's
probably a price worth paying.

The first commit is docs only. I updated the doc pages for extension methods, type classes, and
the grammar.

We should check that semanticDB does the right thing for extension methods

Look for extension methods also directly in the objects that make up
an implicit scope.

We'd have to at least check that there's no use before definition.
Forbid for now, and come back once we support multiple type parameter
clauses.

CI makes more suggestions than local on bootstrapped tests. I am not sure why. It might have to do with
different setups what packages are forced and what not.

The case where a scope contained only extension methods but no
other givens was mishandled.

If a candidate can be both a conversion and an extension, which
happens if it is overloaded, and the extension is not applicable,
try the conversion.

Check files should be only added if they test a tricky error message, typically
one with lots of configurable parts. A simple string error message should never
lead to a check file. Having too many check file just causes unnecessary friction for
me and the other maintainers.

…uments

There's will probably be a tendency to put a `:` in a collective extension
by analogy to given and object, trait etc. We don't need to be picky about
this.