Add StartOfTextToggle and SelectToStartOfTextToggle actions. (#1612)

These actions reintroduce the behavior of micro where the Home key
toggles between the start of text (first) and the start of the line.
The same applies for the variant with selection. This commit also
sets these bindings as the defaults.

internal/buffer/cursor.go

@@ -109,6 +109,19 @@ func (c *Cursor) StartOfText() {
 	}
 }
 
+// IsStartOfText returns whether the cursor is at the first
+// non-whitespace rune of the line it is on
+func (c *Cursor) IsStartOfText() bool {
+	x := 0
+	for util.IsWhitespace(c.RuneUnder(x)) {
+		if x == utf8.RuneCount(c.buf.LineBytes(c.Y)) {
+			break
+		}
+		x++
+	}
+	return c.X == x
+}
+
 // End moves the cursor to the end of the line it is on
 func (c *Cursor) End() {
 	c.X = utf8.RuneCount(c.buf.LineBytes(c.Y))