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.

runtime/help/keybindings.md

@@ -162,6 +162,8 @@ SelectUp
 SelectDown
 SelectLeft
 SelectRight
+SelectToStartOfText
+SelectToStartOfTextToggle
 WordRight
 WordLeft
 SelectWordRight
@@ -209,6 +211,8 @@ HalfPageUp
 HalfPageDown
 StartOfLine
 EndOfLine
+StartOfText
+StartOfTextToggle
 ParagraphPrevious
 ParagraphNext
 ToggleHelp
@@ -245,6 +249,9 @@ JumpToMatchingBrace
 Autocomplete
 ```
 
+The `StartOfTextToggle` and `SelectToStartOfTextToggle` actions toggle between
+jumping to the start of the text (first) and start of the line.
+
 You can also bind some mouse actions (these must be bound to mouse buttons)
 
 ```
@@ -410,21 +417,21 @@ conventions for text editing defaults.
     "ShiftDown":      "SelectDown",
     "ShiftLeft":      "SelectLeft",
     "ShiftRight":     "SelectRight",
-    "AltLeft":        "WordLeft",
-    "AltRight":       "WordRight",
+    "AltLeft":        "WordLeft", (Mac)
+    "AltRight":       "WordRight", (Mac)
     "AltUp":          "MoveLinesUp",
     "AltDown":        "MoveLinesDown",
     "CtrlShiftRight": "SelectWordRight",
     "CtrlShiftLeft":  "SelectWordLeft",
-    "AltLeft":        "StartOfLine",
+    "AltLeft":        "StartOfTextToggle",
     "AltRight":       "EndOfLine",
     "AltShiftRight":  "SelectWordRight", (Mac)
     "AltShiftLeft":   "SelectWordLeft", (Mac)
     "CtrlLeft":       "StartOfText", (Mac)
     "CtrlRight":      "EndOfLine", (Mac)
-    "AltShiftLeft":   "SelectToStartOfLine",
-    "CtrlShiftLeft":  "SelectToStartOfText", (Mac)
-    "ShiftHome":      "SelectToStartOfLine",
+    "AltShiftLeft":   "SelectToStartOfTextToggle",
+    "CtrlShiftLeft":  "SelectToStartOfTextToggle", (Mac)
+    "ShiftHome":      "SelectToStartOfTextToggle",
     "AltShiftRight":  "SelectToEndOfLine",
     "CtrlShiftRight": "SelectToEndOfLine", (Mac)
     "ShiftEnd":       "SelectToEndOfLine",