Select: add showSelectionIndicator option

[leonid-shutov]

Issue #1193

Summary

Add a showSelectionIndicator boolean option to SelectRenderable that controls whether the ▶ glyph is rendered next to the currently selected item.

API

const select = new SelectRenderable(renderer, {
  width: 40,
  height: 10,
  options,
  showSelectionIndicator: false, // hides ▶ and reclaims its 2-column gutter
})

Default: true (no behaviour change for existing consumers).

When set to false, the indicator glyph and its 2-column gutter are both removed, giving item text two extra columns of horizontal space.

Screenshots

Incidental cleanup

Two refactors that aren't part of the feature but were needed to keep the render loop coherent once the configurable indicator was introduced:

1. Removed const contentX = 0 / const contentY = 0. Always 0, added as-is to every coordinate (a no-op).
2. Renamed descX → textX. It was seeded as the description's x-position but then reused as the start column for the item name too.