Add support for `if-let chain`

[faptc]

if-let chain will soon be stable. Tracking issue for the feature: https://github.com/rust-lang/rust/issues/53667

The syntax is not highlighted correctly for the let keyword.

I have this code:

fn foo(b: bool, x: Option<u32>) {
    if b && let Some(x) = x {}
}

I expect the let keyword be highlighted.

Instead, let is not highlighted:

[AndyJado]

perhaps let-else as well?

[solidiquis]

Yeah let-else and if-let chain would be nice.