Move external dependencies to node_modules

[cherniavskii]

This PR removes hardcoded versions of expect and sinon and adds them to devDependencies

[ghybs]

Hi,

I like the cleanup! 👍

For history purpose, expect and sinon were initially hard-coded as was done for jasmine, in order to easily launch the spec/index.html test file in a browser without npm install prerequisite.
See PR #1479

However, right from the evolution of that PR, other libs were included, using npm dependencies.
Therefore there is absolutely no reason keeping these 2 libs hard-coded instead of using them through dependencies (i.e. through node_modules).

These hard-coded versions were:

• expect: 0.1.2, replaced by ^0.3.1 in this PR (through karma-expect version ^1.1.3) (current latest 1.20.2)
• sinon: 1.6.0, replaced by ^1.6.0 in this PR (current latest 4.2.2).

[ghybs]

LGTM, tests pass

[ghybs]

BTW we should also mention that the leaflet-include.js file / symlink is no longer relevant since the shift to Rollup build: browsers no longer understand modules with import and export (except through new <script type="module"> syntax, but support is still narrow).

Therefore we now need to build the dist file in order to be able to launch tests from the spec/index.html page.
Hence replacing leaflet-include.js by dist/leaflet-src.js directly is absolutely appropriate.