support lts-20

[juhp]

I am trying to build tart with roughly lts-20: actually for Fedora 38. One problem is likely that lts now has brick-1.4, however even before hitting that I get:

programs/Theme.hs:30:13: error:
    • No instance for (Data.String.IsString AttrName)
        arising from the literal ‘"error"’
    • In the expression: "error"
      In an equation for ‘errorAttr’: errorAttr = "error"
   |
30 | errorAttr = "error"
   |             ^^^^^^^

Maybe some upperbound missing?

The library builds just fine.