On start support (#324)

* annotating APIs to support onstart

* missed file

* updated templates

* adding new lines in main.ts

* fixing docs

* removing onstart

* updated docs

* updated various docs

* more docs

* adding upgrade policies for blocks

* updated pxt reference

* placing on start under "basic"

docs/blocks/on-start.md

@@ -0,0 +1,22 @@
+# On Start
+
+An event that runs when the program starts.
+
+The ``on start`` is a special event that runs when the program starts, before any other event. 
+Use this event to initialize your program.
+
+## Example
+
+In this example, ``on start`` sets a dimmer brightness on the screen and the button handler shows a string.
+
+```blocks
+input.onButtonPressed(Button.A, () => {
+    basic.showString("Hello!")
+})
+led.setBrightness(50)
+```
+
+
+## What about JavaScript?
+
+``on-start`` only exists in the block editor. In JavaScript, all code executes sequentially from the first line.