lua/stdlib: Introduce vim.shared

This is where "pure functions" can live, which can be shared by Nvim and
test logic which may not have a running Nvim instance available.

If in the future we use Nvim itself as the Lua engine for tests, then
these functions could be moved directly onto the `vim` Lua module.

closes #6580

runtime/lua/vim/shared.lua

@@ -0,0 +1,39 @@
+--- Shared functions
+--    - Used by Nvim and tests
+--    - Can run in vanilla Lua (do not require a running instance of Nvim)
+
+
+--- Merge map-like tables.
+--
+--@see |extend()|
+--
+-- behavior: Decides what to do if a key is found in more than one map:
+--           "error": raise an error
+--           "keep":  skip
+--           "force": set the item again
+local function tbl_extend(behavior, ...)
+  if (behavior ~= 'error' and behavior ~= 'keep' and behavior ~= 'force') then
+    error('invalid "behavior": '..tostring(behavior))
+  end
+  local ret = {}
+  for i = 1, select('#', ...) do
+    local tbl = select(i, ...)
+    if tbl then
+      for k, v in pairs(tbl) do
+        if behavior ~= 'force' and ret[k] ~= nil then
+          if behavior == 'error' then
+            error('key found in more than one map: '..k)
+          end  -- Else behavior is "keep".
+        else
+          ret[k] = v
+        end
+      end
+    end
+  end
+  return ret
+end
+
+local module = {
+  tbl_extend = tbl_extend,
+}
+return module