Previously variable declaration required a keyword, e.g.
```kcl
let x = 4
const x = 4
var x = 4
```
These were all valid, and did the exact same thing. As of this PR, they're all still valid, but the KCL formatter will change them all to just:
```kcl
x = 4
```
which is the new preferred way to declare a constant.
But the formatter will remove the var/let/const keywords.
Closes https://github.com/KittyCAD/modeling-app/issues/3985
