Allow specifying max lines used for AI code completion context

[JonasHelming]

Review #14537 first, please

What it does

Adds a new user setting allowing to specify max lines used for AI code completion context

How to test

• Change the setting to other values than 0
• Open the AI History View => Code Completion
• Trigger code completion (SHIFT+SPACE) and check the context

Follow-ups

This could be more sophisticated, e.g. with percentages for prefix and suffix or by respecting the AST.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[martin-fleck-at]

The change looks good to me and works as expected. Thank you very much, Jonas! However, I do have some questions regarding the setting as a whole that I'd like to clarify before we merge this.

[martin-fleck-at]

I had another look and everything works really nicely! Thank you very much for the quick turnaround!