Support /// for documentation comments

We should consider supporting `///` for use in documentation comments.

Approximate spec:

A doc comment is extended to support a series of adjacent lines each beginning with optional whitespace followed by exactly `///` ... with the rest of the line being comment content.

See also the spec in Elements.getDocComment.

We should also consider a new method alongside Elements.getDocComment to provide the "style" of the comment (`/**` or `///`)

As with `/**` comments, `DocSourcePositions` should be able to map positions within a `///` comment to positions in the enclosing source file.