* feat(lua): `Range:is_empty()` to check vim.range emptiness
* fix(lsp): don't overlay insertion-style inline completions
**Problem:** Some servers commonly respond with an empty inline
completion range which acts as a position where text should be inserted.
However, the inline completion module assumes that all responses with a
range are deletions + insertions that thus require an `overlay` display
style. This causes an incorrect preview, because the virtual text should
have the `inline` display style (to reflect that this is purely an
insertion).
**Solution:** Only use `overlay` for non-empty replacement ranges.
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