Update the library to use ES6 modules

[daniel-grabowski]

• [x] briefly describe the changes in this PR
• [ ] write tests for all new functionality
• [x] run npm run docs and commit changes to API.md
• [x] update CHANGELOG.md with changes under master heading before merging

Changes:

• Removed all entries of 'use strict'.
• Converted require(...) to import.
• Converted module.exports to export.
• Added the browserify plugin esmify to allow compatibility between ES6 modules and commonjs modules for debug and unit tests scripts.
• Upgraded the library sinon to ^15.0.3 to make it compatible with ES6. The tests were failing using the older version.
• Since there were no functionality changes, no new unit tests were created. All previous unit tests are passing.
• All three debug scripts are working: debug/index.js, debug/filter.js and debug/nomap.js.

Since using require to load an ES module is not supported, required code will need to change from require(...) to import.

Related to https://github.com/mapbox/mapbox-gl-geocoder/issues/429