feat: Optimize source generation by using GetText

[faddiv]

Optimize source generation by using GetText

Description

This is a small improvement I wished to send in for a while:
This way of the source text generation is better, since it internally contains a decision: if the output size is larger than the Large Object Heap limit, then it uses LargeTextWriter, which breaks the source apart, so the LOH allocation is avoided. On the benchmark, there is no visible change, but I think it is worth it.

Fixes # (issue)

Checklist

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

[latonz]

Definitely worth it, thanks for the contribution! ❤️