New external plugins repositories

[wa0x6e]

Following #366

The current plugin files are not exported correctly. I made some tests, by trying to add all plugins files and types to the export property in package.json, and got it working only on when moduleResolution is set to Node16 in tsconfig, and type set to module in package.json in the package importing cal-heatmap.

Multiple export from package.json is available only on recent config, and broken on package using node, or esnext. To ensure the broadest support of setup, multiple export is not a solution.

This left us with 2 solutions:

1. merge the plugins inside the core package, and import them with import { Tooltip } from 'cal-heatmap' => will increase build size
2. externalize the plugins to their own NPM package => will require installing each plugin as a new dependency

Solution 2 has been retained, and all plugins will have their own repositories in https://github.com/cal-heatmap/

Refactoring is in progress, and next release (4.3.0) will incorporate this breaking changes.