feat(lua): allow :=expr as a shorter version of :lua =expr

existing behavior of
  :=
and
  :[range]=

are unchanged. `|` is still allowed with this usage.

However,
   :=p
and similar are changed in a way which could be construed as a breaking
change. Allowing |ex-flags| for := in the first place was a mistake as
any form of := DOES NOT MOVE THE CURSOR. So it would print one line number
and then print a completely different line contents after that.

src/nvim/cmdexpand.c

@@ -2141,6 +2141,7 @@ static const char *set_context_by_cmdname(const char *cmd, cmdidx_T cmdidx, expa
     return set_context_in_scriptnames_cmd(xp, arg);
 
   case CMD_lua:
+  case CMD_equal:
     xp->xp_context = EXPAND_LUA;
     break;
 

src/nvim/ex_cmds.lua

@@ -3319,7 +3319,7 @@ module.cmds = {
   {
     command='=',
     enum='CMD_equal',
-    flags=bit.bor(RANGE, TRLBAR, DFLALL, FLAGS, CMDWIN, LOCK_OK),
+    flags=bit.bor(RANGE, EXTRA, DFLALL, ARGOPT, CMDWIN, LOCK_OK),
     addr_type='ADDR_LINES',
     func='ex_equal',
   },

src/nvim/ex_docmd.c

@@ -5658,8 +5658,13 @@ static void ex_pwd(exarg_T *eap)
 /// ":=".
 static void ex_equal(exarg_T *eap)
 {
-  smsg("%" PRId64, (int64_t)eap->line2);
-  ex_may_print(eap);
+  if (*eap->arg != NUL && *eap->arg != '|') {
+    // equivalent to :lua= expr
+    ex_lua(eap);
+  } else {
+    eap->nextcmd = find_nextcmd(eap->arg);
+    smsg("%" PRId64, (int64_t)eap->line2);
+  }
 }
 
 static void ex_sleep(exarg_T *eap)

src/nvim/lua/executor.c

@@ -1621,14 +1621,15 @@ void ex_lua(exarg_T *const eap)
     xfree(code);
     return;
   }
-  // When =expr is used transform it to print(vim.inspect(expr))
-  if (code[0] == '=') {
-    len += sizeof("vim.print()") - sizeof("=");
+  // When =expr is used transform it to vim.print(expr)
+  if (eap->cmdidx == CMD_equal || code[0] == '=') {
+    size_t off = (eap->cmdidx == CMD_equal) ? 0 : 1;
+    len += sizeof("vim.print()") - 1 - off;
     // code_buf needs to be 1 char larger then len for null byte in the end.
     // lua nlua_typval_exec doesn't expect null terminated string so len
     // needs to end before null byte.
     char *code_buf = xmallocz(len);
-    vim_snprintf(code_buf, len + 1, "vim.print(%s)", code + 1);
+    vim_snprintf(code_buf, len + 1, "vim.print(%s)", code + off);
     xfree(code);
     code = code_buf;
   }