Add `collapsible_if`

[0xLucqs]

What it does

Checks for nested if statements which can be collapsed by &&-combining their conditions.

Why is this bad?

Each if-statement adds one level of nesting, which makes code look more complex than it really is.

Example

if x {
    if y {
        // …
    }
}

Use instead:

if x && y {
    // …
}

[jimenezz22]

gm @0xLucqs I could help with this issue

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