settings_ui: Add pickers for theme and icon themes (#40829)

In the process of adding pickers for the theme and icon themes fields in
the settings UI, I felt like there was an improvement opportunity in
regards to where some of these components are stored. The `ui_input`
crate originally was meant only for the text field-like component, which
couldn't be in the regular `ui` crate due to the dependency with
`editor`. Given we had also added the number field there—which is
similar in also having the same dependency—it made sense to think of
this crate more like a home for form-like components rather than for
only one component.

However, we were also storing some settings UI-specific stuff in that
crate, which didn't feel right. So I ended up creating a new directory
within the `settings_ui` for components and moved all the pickers and
the custom input field there. I think this makes it for a cleaner
structure.

Release Notes:

- settings_ui: Added the ability to search for theme and icon themes in
their respective fields.

Cargo.lock

@@ -10788,7 +10788,6 @@ dependencies = [
  "telemetry",
  "theme",
  "ui",
- "ui_input",
  "util",
  "vim_mode_setting",
  "workspace",
@@ -15271,6 +15270,7 @@ dependencies = [
  "menu",
  "node_runtime",
  "paths",
+ "picker",
  "pretty_assertions",
  "project",
  "schemars 1.0.4",
@@ -18191,10 +18191,8 @@ version = "0.1.0"
 dependencies = [
  "component",
  "editor",
- "fuzzy",
  "gpui",
  "menu",
- "picker",
  "settings",
  "theme",
  "ui",