9600: fix: Single-line and nested blocks in the `unwrap_block` assist r=Veykril a=patrick-gu
Fixes#8411
Rework the system for stripping whitespace and braces in the unwrap_block assist to allow correct unwrapping of blocks such as:
```rust
{ $0 0 }
```
into
```rust
0
```
and nested blocks, such as:
```rust
$0{
{
3
}
}
```
into
```rust
{
3
}
```
This is done by creating the `update_expr_string_with_pat` function (along with `update_expr_string` and `update_expr_string_without_newline`), which strips whitespace and braces in a way that ensures that only whitespace and a maximum of one brace are removed from the start and end of the expression string.
I have also created several tests to ensure that this functionality works correctly.
Co-authored-by: patrick-gu <55641350+patrick-gu@users.noreply.github.com>
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