[Discover] Move truncate-by-height into Discover

[timroes]

We currently have a truncate-by-height CSS class we're only using in Discover and the source field formatter in kibanaLegacy. We should restructure htis a bit, to help getting rid of the kibanaLegacy plugin (see #76905):

• Move _source field formatter into Discover and have that code inline, the same as we do for the new data grid implementation.
• Move the truncate-by-height CSS code into Discover and rewrite it to emotion
• Move the regristration of the truncate:maxHeight setting from Core into the Discover plugin and also put it into the Discover category there
• Change documentation and description for the advanced setting to make it more clear it's only affecting Discover

cc @flash1293 @stratoula

[kertal]

Here's the PR enabling emotion in Kibana: #98157

And an example how it's used:

kibana/src/plugins/chart_expressions/expression_tagcloud/public/expression_renderers/tagcloud_renderer.tsx

Line 54 in eef094b

<ClassNames>