Tilde not concealed in column headers

[clason]

E.g., for :h api-types, the first table has a column heading

Basic types ~

which corresponds to the playground

block [94, 0] - [107, 0]
  line [94, 0] - [95, 0]
    column_heading [94, 0] - [95, 0]
      name: word [94, 0] - [94, 5]
      name: word [94, 6] - [94, 11]

Since ~ does not appear as an (anonymous) node, it cannot be captured and thus concealed as in the legacy syntax.

This is the same issue (with the same likely fix) as in #33.