feat(vim.version): make tostring() return human-readable version range

Problem: `tostring()` applied to version range doesn't return
human-readable text with information about the range.

Solution: Add `__tostring()` method.

runtime/doc/news.txt

@@ -199,6 +199,7 @@ LUA
 • |vim.hl.range()| now allows multiple timed highlights.
 • |vim.tbl_extend()| and |vim.tbl_deep_extend()| now accept a function behavior argument.
 • |vim.fs.root()| can define "equal priority" via nested lists.
+• |vim.version.range()| output can be converted to human-readable string with |tostring()|.
 
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