feat: improve constructor mapping readability

[aradalvand]

Fixes #1322

It's my first time contributing to a source generator project, so feel free to let me know if I've made any obvious mistakes. Thanks.

Checklist

• [x] The existing code style is followed
• [x] The commit message follows our guidelines
• [x] Performed a self-review of my code
• [x] Hard-to-understand areas of my code are commented
• [x] The documentation is updated (as applicable)
• [x] Unit tests are added/updated
• [x] Integration tests are added/updated (as applicable, especially if feature/bug depends on roslyn or framework version in use)

[aradalvand]

@latonz Thanks! Sorry I've been very busy and couldn't get to fixing those, will do so this weekend when I hopefully get the time.

[latonz]

This received no updates for a longer time, I have done the remaining work and created a new PR https://github.com/riok/mapperly/pull/1448. Thanks for your contribution!