Fixes in completion and interactive specs

[vkazanov]

Fixes extrated from #51:

1. Fix completion by using new functions.
2. Silence the compiler on 26.1 by fixing the interactive spec.

I feel that the completion-at-point logic is too fragile and needs more unit tests.

Thanks!

[vkazanov]

Having looked at (beancount-completion-at-point) I am not even sure that tag and link completion can work at all.

The code in the completion function does roughly the following:

1. go to beginning of line
2. check if the line matches a regexp

The branch in cond that matches for tags and links has the following check:

(beancount-looking-at (concat "[ \t]+#\\\\([" beancount-tag-chars "]*\\\\)") 1 pos)

Note how this is describing a whitespace followed by a tag. But tags can only happen at the end of a transaction-defining line.