core.formspec_escape: Restore backwards compat

Support numbers as arguments by using `string.gsub(text, ...)` instead of `text:gsub(...)` which will coerce `text` to a string

builtin/common/misc_helpers.lua

@@ -254,7 +254,7 @@ local formspec_escapes = {
 }
 function core.formspec_escape(text)
  -- Use explicit character set instead of dot here because it doubles the performance
- return text and text:gsub("[\\%[%];,]", formspec_escapes)
+ return text and string.gsub(text, "[\\%[%];,]", formspec_escapes)
 end