patch 8.2.4183: cannot use an import in 'formatexpr'

Problem:    Cannot use an import in 'formatexpr'.
Solution:   Set the script context when evaluating 'formatexpr'.
diff --git a/src/textformat.c b/src/textformat.c
index 3c343cd..fd0d31d 100644
--- a/src/textformat.c
+++ b/src/textformat.c
@@ -903,6 +903,7 @@
 								   OPT_LOCAL);
     int		r;
     char_u	*fex;
+    sctx_T	save_sctx = current_sctx;
 
     // Set v:lnum to the first line number and v:count to the number of lines.
     // Set v:char to the character to be inserted (can be NUL).
@@ -914,6 +915,7 @@
     fex = vim_strsave(curbuf->b_p_fex);
     if (fex == NULL)
 	return 0;
+    current_sctx = curbuf->b_p_script_ctx[BV_FEX];
 
     // Evaluate the function.
     if (use_sandbox)
@@ -924,6 +926,7 @@
 
     set_vim_var_string(VV_CHAR, NULL, -1);
     vim_free(fex);
+    current_sctx = save_sctx;
 
     return r;
 }