chore!: reserve unchecked keyword (#4432)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

There's a decent chance that we're going to add some form of unchecked
maths in future so I'd like to reserve this keyword in preparation for
that.

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[Exceptional Case]** Documentation to be submitted in a separate
PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

compiler/noirc_frontend/src/lexer/token.rs

@@ -673,6 +673,7 @@ pub enum Keyword {
     Struct,
     Trait,
     Type,
+    Unchecked,
     Unconstrained,
     Use,
     Where,
@@ -715,6 +716,7 @@ impl fmt::Display for Keyword {
             Keyword::Struct => write!(f, "struct"),
             Keyword::Trait => write!(f, "trait"),
             Keyword::Type => write!(f, "type"),
+            Keyword::Unchecked => write!(f, "unchecked"),
             Keyword::Unconstrained => write!(f, "unconstrained"),
             Keyword::Use => write!(f, "use"),
             Keyword::Where => write!(f, "where"),
@@ -760,6 +762,7 @@ impl Keyword {
             "struct" => Keyword::Struct,
             "trait" => Keyword::Trait,
             "type" => Keyword::Type,
+            "unchecked" => Keyword::Unchecked,
             "unconstrained" => Keyword::Unconstrained,
             "use" => Keyword::Use,
             "where" => Keyword::Where,