Add highlighting for identifiers in nested scope

[sapek]

Defined hsIdentifierNested syntax highlighting item for identifiers
defined in where clause. It matches identifier preceded by whitespaces
when it is followed by = on the same line or the next line starting with
whitespace preceded |. The = is not considered when it is within {} to
handle syntax like:

    Foo {foo = bar} <- action

or

    foo (\x -> x {foo = bar})

By default hsIdentifierNested is highlighted as Identifier but it is
a distinct item from hsIdentifier to allow not highlighting it or
using a different color than for top level identifiers.