fix(editor): keep selections whole over line backgrounds and indent guides (#1568)

* fix(editor): keep selections whole over line backgrounds and indent guides

Multi-line selections could show a 1px seam between their stacked
rectangles, indent guides showed as short dashes inside a selection,
and a breakpoint or execution line hid the selection past its text.

- Snap selection rectangle edges to device pixels so stacked pieces
  share one edge.
- Paint caret-row, breakpoint and execution line backgrounds on a
  pseudo-element below the selection layer.
- Draw indent guides on a pseudo-element above the text.

* test(e2e): check the visible selection layer in the theme flow

src/features/debugger/editor/breakpointGutter.ts

@@ -84,7 +84,7 @@ class BreakpointMarker extends GutterMarker {
   }
 }
 
-const bpLine = Decoration.line({ class: styles.bpLine });
+const bpLine = Decoration.line({ class: `${styles.bpLine} cm-openidea-lineBg` });
 const DISABLED_LOOKS = new Set<BreakpointLook>(["disabled", "muted", "methodDisabled", "fieldDisabled"]);
 
 const setMarkers = StateEffect.define<RangeSet<BreakpointMarker>>();