Disallow single-line `IF expr ELSE` without `THEN`

* Supplement missing block before `ELSE` token only for multi-line `if`.
* Don't prematurely remove `TERMINATOR` before `ELSE` (so we can  differentiate single- and multi-line forms).
* Cleanup: Remove `WHEN` from `EXPRESSION_CLOSE`. (Only `LEADING_WHEN` tokens are preceded by a `TERMINATOR`.)
* Cleanup: Remove really old, inapplicable text from comment.
Fix multi-line if-else in single-line expression.

* When searching for the closing token of a single-line expression, ignore TERMINATORS that will subsequently be removed.
* Add a test.
Merge `removeMidExpressionNewlines` into `addImplicitIndentation`

and rename it to `normalizeLines`
Merge pull request #3096 from marchaefner/issue2994

Disallow single-line `IF expr ELSE` without `THEN`