Add high level API (#160)

* Add high-level api extensions for BleDevice and Characteristic

* Add doc comments and move few files to utils

* Add advance documentation

* Make the high level APIs the new default in the readme

* More readme cleanups

* Add pair api, improve notifications apis and deprecate writeValue

* Use highLevel apis in Example

* Rename HighLevel isPaired api to hasPairing api

* Update Readme

* Rename low level readValue api to read and update Readme

* Update UniversalBle to use new apis instead of deprecated internally

* Split queueCommand and queueCommandWithoutTimeout

* Split setNotifiable into subscribeNotifications, subscribeIndications and unsubscribe

* Fix web imports

* Update changelog

* Switch back to isPaired

Co-authored-by: Rohit Sangwan <rohitsangwan647@gmail.com>

* Fix BleDevice toString method

* Switch back to isPaired

* Change peripheral to device

* Rename cached to preferCached

* Remove preferCached from discoverServices()

* Update changelog

* Use cached services

* Add ///

* Reverse withoutResponse to withResponse to avoid negation

* Link to low level API

* Move connectionState to public readme

* Rename disableSubscriptions to unsubscribe

* Improve documentation

* Fix getService cache

* Add NotFoundException

---------

Co-authored-by: Foti Dim <foti@navideck.com>

example/pubspec.lock

@@ -402,7 +402,7 @@ packages:
       path: ".."
       relative: true
     source: path
-    version: "0.19.1"
+    version: "0.20.0"
   vector_math:
     dependency: transitive
     description: