feat(concealer): code block background min_width

[benlubas]

closes #1327

It doesn't quite do what that issue says, but I like this better.

Lets you set a min width for code cell background highlights. It's also an incredibly simple change.

Example

New configuration looks like this:

-- ...
["core.concealer"] = {
  config = {
    icons = {
      code_block = {
        width = "content",
        min_width = 85,
      },
    },
  },
},
-- ...

And the result: