Continue from https://github.com/discourse/discourse/pull/25673.
This commit starts building the inputs pane of schema theme settings. At the moment only string fields are rendered, but more types will be added in future commits.
This commit is the first of a series of commits that will allow themes to define complex settings types by declaring a schema of the setting structure that Discourse core will use to build a UI for the setting automatically. We implement the navigation logic and support for multiple levels of nesting in this commit and we'll continue building this new system gradually in future commits.
Internal topic: t/116870.
