Consolidate JS SDK packages

[adams85]

Describe the purpose of your pull request

The initial state of this repo is a copy of https://github.com/configcat/common-js. This PR merges the following repos containing platform-specific JS SDKs into that code base:

• https://github.com/configcat/node-sdk
• https://github.com/configcat/js-sdk
• https://github.com/configcat/js-ssr-sdk
• https://github.com/configcat/js-chromium-extension-sdk

The goal is to end up with a single NPM package that can be used as a drop-in replacement for the listed packages.

This will allow us to eliminate a lot of redundancy (especially w.r.t. build configuration and documentation), improve development velocity and, thus, significantly reduce maintenance costs. As a side effect, the unified package will also provide a better support for SSR applications.

Additional benefits and improvements:

• We can drop the axios dependency as it is not needed anymore. The platform-specific targeting is already handled via the main / browser and exports package.json fields.
• The tests specified in common-js have only been run on Node.js so far. Now we can run them on all the target platforms, which increases reliability.
• Tests have used the source code (src) so far instead of the compiled code (lib/dist). The build configuration have been adjusted so now we test the code we actually distribute.
• New dist bundles have been added, so consumers who don't want to support ancient browsers can use more lightweight builds of the library.
• The intermediate lib builds for Node.js and bundlers now targets ES2017, which allows consumers to choose the degree of downleveling, which can save many kilobytes if support for old browsers is not necessary.
• This also allows us to drop the tslib dependency, which in turn can also reduce application bundle sizes.
• A few nasty non-deterministic (GC-related) bugs have been fixed in the tests.
• Test running in the browser has been revised and cleaned up, which made it possible to get rid of some problematic dependencies. Now the code coverage collection and aggregation is entirely based on istanbuljs.
• Sample apps have been updated and a bunch of new sample apps have been added. Thanks to this, the package have also been tested with the widely used bundlers.
• Support for Deno.
• Minor performance improvements (execution and bundle size).

Requirement checklist (only if applicable)

• I have covered the applied changes with automated tests.
• I have executed the full automated test set against my changes.
• I have validated my changes against all supported platform versions.
• I have read and accepted the contribution agreement.

[sonarqubecloud]

Quality Gate passed

Issues
11 New issues
0 Accepted issues

Measures
0 Security Hotspots
85.3% Coverage on New Code
1.1% Duplication on New Code

See analysis details on SonarCloud

[z4kn4fein]

Just 2 small suggestions, but overall, it looks good to me!

[z4kn4fein]

I'd remove this link from the top of the README (it'll be in the About section of the repo anyway)

[z4kn4fein]

I'd move this to the end of the readme under an About ConfigCat section. IMO this part of the README should be about the repo's content.

[sonarqubecloud]

Quality Gate passed

Issues
22 New issues
0 Accepted issues

Measures
0 Security Hotspots
84.1% Coverage on New Code
2.5% Duplication on New Code

See analysis details on SonarCloud