Add configurable minimum hidden-line threshold for fold placeholders

When collapsing a very small folding region, VS Code displays a placeholder such as 1 hidden line.

For very small folds, this **adds visual noise without providing much useful information**. I'd like to be able to configure a minimum number of hidden lines before the fold placeholder is rendered.

For example:
`"editor.foldingMinimumLines": 2`
A threshold of 0 could preserve the current behavior.