feat: ✨ add gatthrm bootloader code

apps/bootgatthrm/ChangeLog

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+0.01: Initial release.

apps/bootgatthrm/README.md

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+# BLE GATT Battery Service
+
+Adds the GATT Battery Service to advertise the percentage of battery currently remaining over Bluetooth.
+
+## Usage
+
+This boot code runs in the background and has no user interface.
+
+## Requests
+
+If you have any suggestions or ideas please post in [this forum thread](http://forum.espruino.com/conversations/351959/),
+or [@jjok](https://github.com/jjok) in your Github issue.
+
+## Creator
+
+[Jonathan Jefferies](https://github.com/jjok)

apps/bootgatthrm/boot.js

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+(() => {
+  function setupHRMAdvertising() {
+    /*
+     * This function preparse BLE heart rate Advertisement.
+     */
+    NRF.setServices({
+      0x180D: { // heart_rate
+        0x2A37: { // heart_rate_measurement
+          notify: true,
+          value: [0x06, 0],
+        }
+      }
+    }, { advertise: ['180D'] });
+
+  }
+
+  function updateBLEHeartRate(hrm) {
+    /*
+     * Send updated heart rate measurement via BLE
+     */
+    if (hrm === undefined) return;
+    try {
+      NRF.updateServices({
+        '180d': {
+          '2a37': {
+            value: [
+              0x06, //
+              hrm
+            ],
+            notify: true
+          }
+        }
+      });
+    } catch (error) {
+      // After setupHRMAdvertising() BLE needs to restart.
+      // We force a disconnect if the Bangle was connected while setting HRM
+      NRF.disconnect();
+    }
+  }
+
+  setupHRMAdvertising();
+  Bangle.on("HRM", function (hrm) { updateBLEHeartRate(hrm.bpm); });
+})();

apps/bootgatthrm/metadata.json

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+{
+  "id": "bootgatthrm",
+  "name": "BLE GATT HRM Service",
+  "shortName": "BLE HRM Service",
+  "version": "0.01",
+  "description": "Adds the GATT HRM Service to advertise the measured HRM over Bluetooth.\n",
+  "icon": "bluetooth.png",
+  "type": "bootloader",
+  "tags": "hrm,health,ble,bluetooth,gatt",
+  "supports": ["BANGLEJS","BANGLEJS2"],
+  "readme": "README.md",
+  "storage": [
+    {"name":"gatthrm.boot.js","url":"boot.js"}
+  ]
+}