Remove trailing paragraph/interblock separators in right-open slice intervals

[Witiko]

As discovered by @lostenderman during their work on #403, using right-open slice intervals, such as sliced-div ^first-heading in test file lunamark-markdown/slice-fenced-divs.test, causes trailing paragraph/interblock separators to be produced at the end of the document. This may lead to rendering artefacts, since no separators would naturally be produced at the end of a document without the use of slicing.

Acceptance criteria

A pull request that closes this ticket should do the following:

• Remove trailing paragraph/interblock separators in test files lunamark-markdown/*slice*.test.
• Update the parser, so that continuous integration passes.