ISLE feature request: `when` and `unless`

#### Feature

Add `(when )` and `(unless )` syntax as shorthand for `(if-let $true )` and `(if-let $false )` respectively

#### Benefit

Less to type when adding boolean conditions to rules

#### Implementation

Should be simple to implement as a syntax sugar rewrite in the ISLE compiler. I would be happy to implement it myself.

#### Alternatives

Leave ISLE unchanged