`taxy-magit-section-insert` should not signal error when passed a `taxy` struct

The documented examples suggest that `taxy-magit-section-pp` can accept a `taxy` struct. However, `taxy-magit-section-insert`'s labeled function `insert-taxy` attempts to access the `level-indent` slot of the `taxy` argument.

#15 was an attempt to resolve this issue, but since the `insert-taxy` labeled function has conditional logic based on the type of the `taxy` argument, it's has been closed.