[FL-3925, FL-3942, FL-3944] JS features & bugfixes (SDK 0.2) (#4075)

* feat: JS GPIO PWM, JS GUI Widget view; fix: JS EvtLoop stop on request, JS EvtLoop stop on error
* fix: f18 build
* docs: widget
* fix: js unit test
* change feature naming

Co-authored-by: あく <alleteam@gmail.com>

documentation/js/js_gui__widget.md

@@ -0,0 +1,25 @@
+# js_gui__widget {#js_gui__widget}
+
+# Widget GUI view
+Displays a combination of custom elements on one screen
+
+<img src="widget.png" width="200" alt="Sample screenshot of the view" />
+
+```js
+let eventLoop = require("event_loop");
+let gui = require("gui");
+let widgetView = require("gui/widget");
+```
+
+This module depends on the `gui` module, which in turn depends on the
+`event_loop` module, so they _must_ be imported in this order. It is also
+recommended to conceptualize these modules first before using this one.
+
+# Example
+For an example refer to the `gui.js` example script.
+
+# View props
+This view does not have any props.
+
+# Children
+This view has the elements as its children.