Make indent-according-to-mode continue previous line indentation on empty lines

`indent-according-to-mode` is called in:
1. In `newline-and-indent`.
2. In `electric-indent-mode` after `\n` has been inserted.
3. In `evil-open-above`.
4. In `evil-open-below`.

Each of these cases is same: it is to semantically continue previous line. So we need:
1. If previous line ends with operator, continue expression.
2. If inside parenthesized expression, continue on the level of the expression.
3. Lacking other context continue same indentation block.

Note that `haskell-indentation` already has `haskell-indentation-newline-and-indent` that touches this problem but hard to say how good it is.

Reference: https://github.com/syl20bnr/spacemacs/issues/5010, https://github.com/syl20bnr/spacemacs/issues/621, https://github.com/syl20bnr/spacemacs/issues/3162, https://github.com/haskell/haskell-mode/issues/896

Start by tracing the four callers of indent-according-to-mode: newline-and-indent, electric-indent-mode, evil-open-above, and evil-open-below. Compare their behavior with haskell-indentation-newline-and-indent, then verify that empty-line indentation follows the operator, parenthesized-expression, and surrounding-block cases described in the issue.

Written by the indexing model from the issue text.