Implement RFC 2707 + Parser recovery for range patterns

[Centril]

Implement rust-lang/rfcs#2707.

• Add a new basic syntactic pattern form ast::PatKind::Rest (parsed as .. or DOTDOT) and simplify ast::PatKind::{Slice, Tuple, TupleStruct} as a result.

• Lower ast::PatKind::Rest in combination with the aforementioned PatKind variants as well as PatKind::Ident. The HIR remains unchanged for now (may be advisable to make slight adjustments later).

• Refactor parser.rs wrt. parsing sequences and lists of things in the process.

• Add parser recovery for range patterns of form X.., X..=, X..., ..Y, ..=Y, and ...Y.
  This should make it easy to actually support these patterns semantically later if we so desire.

cc #62254

r? @petrochenkov