patch 8.2.2733: detecting Lua version is not reliable

Problem:    Detecting Lua version is not reliable.
Solution:   Add "vim.lua_version". (Ozaki Kiichi, closes #8080)

runtime/doc/if_lua.txt

@@ -208,6 +208,8 @@ Vim evaluation and command execution, and others.
 				created on demand.  Example: >
 					:lua print(vim.fn.has('timers'))
 <
+	vim.lua_version		The Lua version Vim was compiled with, in the
+				form {major}.{minor}.{patch}, e.g. "5.1.4".
 
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 3. List userdata					*lua-list*