Add `trademark` to level4 of us-intl layout

The trademark symbol is a semantic extension of the latin-1 registered symbol, so having it on the next level up from a key that has registered on level3 is intuitive, and it's already present on some newer layouts such as Bépo and Programmer Dvorak. The character is also present in the C1 controls section of the de-facto Windows 1252 encoding scheme used by websites in lieu of the old fashioned ISO-8859-1 encoding, which makes it unlikely to cause any problems in common interchange.