PHP 8 Support. Minimum version bumped to PHP 7.2+

• PHP 8 Support. Minimum version bumped to PHP 7.2+
• #105 address long existing memory leak when batch generating several graphs
• #103 fixes #88 "Trying to access array offset on value of type null" for CSIM plots.
• #102 #101 remove dev related stuff from autoloader, and examples/tests from exported tarball
• #96 enables led examples which didn't have autoloader's logic yet
• #100 handles #99 (PHP8's way to handle invalid GD image resource)
• #93 handles #92 , a misleading exception message when trying to render invalid text with GD

Dropping support for PHP<5.6

Given PHP 5.6 reached its long announced end of life, we're dropping support for it (albeit you can still use this library as long as you use release ~3.6).

Setting the minimum PHP version at 7.0 allows to require Symfony Dotenv version 4 instead of being pinned to the long outdated 3.6 version.

Closes #68

Compatibility with Kint 1.x

When users are running jpgraph in a project which also uses Kint v1.x, polyfilling this class would break things, as reported in #63.

Now, in config.inc.php we check if Kint has property enabled_mode (in which case it's version 2.x) or method enabled in which case it's Kint v1.x. In either case, the debug mode is set accordingly.

Ensures count is applied on array or countables

As described in #62, the library uses count without checking if the parameter is an array or a countable object.
In versions of PHP prior to v7.1, this would return 0 for null and 1 for non-arrays/non-countables. From 7.2 onwards, this practice will emit an E_WARNING.

On this release, we've implemented a global safe_count function, which in turn will call is_countable (if available, as of PHP 7.3) or a polyfill of it declared if the native function does not exist, then return 0 if the parameter is not countable, or the proper item count otherwise.