Fold constant addition expressions. Handles string concatenation and
addition, both with implicit casting.
For example,
```ts
let x = 1 + 1
let y = "hello " + "world"
```
now becomes
```ts
let x = 2
let y = "hello world"
```
## Extra Goodies
- test(minifier): add `test_snapshot` helper to perform snapshot tests
with `insta`
- up(hir): implement `std::ops::Add` for `NumericValue`
- up(span): impl `TryFrom<Cow<'_, &str>>` for `Atom`
