Improve Timelion multiline expression input UX

[cjcenizal]

As noted in #11972 (comment), the UX we want is:

1. Initially the input is a single-line in height.
2. When the user focuses on the input, it expands slightly to indicate that it can accept multiple lines.
3. As the user adds newlines, the input expands, up to a max height.
4. When the input loses focus, it contracts back to a single line.
5. If the user resizes the input, the above behavior is circumvented.

See 6ba00d2 for a prior implementation and #11972 (comment) for an explanation. See https://codepen.io/vsync/pen/frudD for a prototype of complementary behavior.

This is blocked by a Chrome bug: https://bugs.chromium.org/p/chromium/issues/detail?id=94583. Here's a demo of a JS-based workaround: http://jsfiddle.net/nz8ut/2/.

[cjcenizal]

CC @alexfrancoeur

[timroes]

This is addressed in newer versions of Kibana, since we're using the monaco editor for the timelion input clearly showing the multi line behavior.