Advance cursor to next code chunk after executing last line

[jeroenjanssens]

Summary

When running code line-by-line with Cmd+Enter, executing the last line of a code chunk causes the cursor to "fall out" of the chunk into prose. This is disruptive because the next chunk could be dozens of lines away and the user has to navigate back manually. Worse, if the user keeps pressing Cmd+Enter, the interpreter will try to execute the closing fence or prose text, leading to errors.

With this change, executing the last line of a code chunk moves the cursor to the first line of the next code chunk (if one exists). If there's no next chunk, the cursor lands on the closing fence as before.

This covers three code paths in RunCurrentCommand:

• Source editor (Positron, VS Code with R extension, fallback executors)
• Visual editor, Positron (executeAtPosition path)
• Visual editor, non-Positron knitr (line-by-line path)

The "run entire cell" paths (plain VS Code with Python/R, non-knitr visual editor) are unaffected since they don't do line-by-line execution.

Related

#704 describes the same trigger (Cmd+Enter on the last line exits the chunk), though the author there preferred the cursor to stay put rather than advance. Both behaviors could coexist: this PR handles advancing to the next chunk, while Positron's Alt+Enter (posit-dev/positron#2778) already supports executing without moving the cursor.

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