GlyphSet class Null safety

A class that provides access to every configurable glyph.

This is provided as a class so that individual chunks of code can choose between ascii and unicode glyphs. For example:

import 'package:term_glyph/term_glyph.dart' as glyph;

/// Adds a vertical line to the left of [text].
///
/// If [unicode] is `true`, this uses Unicode for the line. If it's
/// `false`, this uses plain ASCII characters. If it's `null`, it
/// defaults to [glyph.ascii].
void addVerticalLine(String text, {bool unicode}) {
  var glyphs =
      (unicode ?? !glyph.ascii) ? glyph.unicodeGlyphs : glyph.asciiGlyphs;

  return text
      .split('\n')
      .map((line) => '${glyphs.verticalLine} $line')
      .join('\n');
}