Allow joining of case statements in `line_joiner.py`

[MarcelSimader]

Allows for the same joining rules that govern if-then statements to be applied to simple case statements.

Examples

This will allow the common (at least in C switch-case blocks, from my experience) idiom of converting if-else-if chains into simple match-case statements, e.g.:

match format:
    case 'l': return string.ljust(width)
    case 'c': return string.center(width)
    case 'r': return string.rjust(width)

Without the proposed changes, the block above would be formatted like this:

match format:
    case 'l': 
        return string.ljust(width)
    case 'c': 
        return string.center(width)
    case 'r': 
        return string.rjust(width)

---

This is my first time contributing to any open source project, so bear with me. Thanks!

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