fix(ui2): ui2 misinterprets getchar() input as pager_char (CR) #41465

Problem:
`vim.on_key()` callbacks are invoked for keys that `getchar()` consumes,
which never reach the main loop, and there is no way to tell such a key
apart.  With ui2 the `on_key` handler activated after interactive
cmdline, handles it as `pager_char` (thus enters the ui2 pager).

Solution:
Skip `vim.on_key()` callbacks during `getchar()`/`getcharstr()`.

test/functional/lua/vim_spec.lua

@@ -2102,7 +2102,7 @@ describe('lua stdlib', function()
       eq('/', exec_lua([[return _G.ctrl_c_cmdtype]]))
     end)
 
-    it('callback is not invoked recursively #30752', function()
+    it('not invoked recursively #30752', function()
       local screen = Screen.new(60, 10)
       exec_lua([[
         vim.on_key(function(key, typed)
@@ -2140,6 +2140,21 @@ describe('lua stdlib', function()
       ]])
     end)
 
+    it('not invoked for keys consumed by getchar() #40010', function()
+      exec_lua(function()
+        _G.typed = {}
+        vim.on_key(function(_, typed)
+          table.insert(_G.typed, vim.fn.keytrans(typed))
+        end)
+        vim.keymap.set('n', '<M-m>', function()
+          _G.got = vim.fn.getchar()
+        end)
+      end)
+      feed('<M-m><CR>')
+      eq(13, exec_lua('return _G.got'))
+      eq({ '<M-m>' }, exec_lua('return _G.typed'))
+    end)
+
     it('can discard input', function()
       -- discard the first key produced by every other 'x' key typed
       exec_lua [[