## description
this pr adds [match
guards](https://doc.rust-lang.org/reference/expressions/match-expr.html#match-guards)
to match patterns
```nushell
match $x {
_ if $x starts-with 'nu' => {},
$x => {}
}
```
these work pretty much like rust's match guards, with few limitations:
1. multiple matches using the `|` are not (yet?) supported
```nushell
match $num {
0 | _ if (is-odd $num) => {},
_ => {}
}
```
2. blocks cannot be used as guards, (yet?)
```nushell
match $num {
$x if { $x ** $x == inf } => {},
_ => {}
}
```
## checklist
- [x] syntax
- [x] syntax highlighting[^1]
- [x] semantics
- [x] tests
- [x] clean up
[^1]: defered for another pr
5bfec20244