system: define new Locale type

This commit adds a Locale struct that captures language and layout
flow direction for the system. This information can be leveraged
by text shaping and layout code to make better choices.

References: https://todo.sr.ht/~eliasnaur/gio/146
Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

io/system/locale.go

@@ -0,0 +1,59 @@
+package system
+
+// Locale provides language information for the current system.
+type Locale struct {
+	// Language is the BCP-47 tag for the primary language of the system.
+	Language string
+	// Direction indicates the primary direction of text and layout
+	// flow for the system.
+	Direction TextDirection
+}
+
+const (
+	axisShift = iota
+	progressionShift
+)
+
+// TextDirection defines a direction for text flow.
+type TextDirection byte
+
+const (
+	// LTR is left-to-right text.
+	LTR TextDirection = TextDirection(Horizontal<<axisShift) | TextDirection(FromOrigin<<progressionShift)
+	// RTL is right-to-left text.
+	RTL TextDirection = TextDirection(Horizontal<<axisShift) | TextDirection(TowardOrigin<<progressionShift)
+)
+
+// Axis returns the axis of the text layout.
+func (d TextDirection) Axis() TextAxis {
+	return TextAxis((d & (1 << axisShift)) >> axisShift)
+}
+
+// Progression returns the way that the text flows relative to the origin.
+func (d TextDirection) Progression() TextProgression {
+	return TextProgression((d & (1 << progressionShift)) >> progressionShift)
+}
+
+// TextAxis defines the layout axis of text.
+type TextAxis byte
+
+const (
+	// Horizontal indicates text that flows along the X axis.
+	Horizontal TextAxis = iota
+	// Vertical indicates text that flows along the Y axis.
+	Vertical
+)
+
+// TextProgression indicates how text flows along an axis relative to the
+// origin. For these purposes, the origin is defined as the upper-left
+// corner of coordinate space.
+type TextProgression byte
+
+const (
+	// FromOrigin indicates text that flows along its axis away from the
+	// origin (upper left corner).
+	FromOrigin TextProgression = iota
+	// TowardOrigin indicates text that flows along its axis towards the
+	// origin (upper left corner).
+	TowardOrigin
+)