Rewrite to use parser or tokenizer instead of serializing

[samdark]

The idea is further development of https://github.com/yiisoft/composer-config-plugin/pull/117.

Potentially we can avoid serializing by following these steps while merging configs:

1. Tokenize/parse config. Replace all closures, $_ENV references and require with unique placeholders. Original code strings are kept in an array where key is this unique unique placeholder name.
2. Read configs produced during step 1 with PHP. Merge them.
3. Tokenize/parse config produced during step 2. Replace unique placeholders with strings saved during step 1.

Advantages

• No need to serialize anything.
• Will likely work well with PHP 8.
• No need to depend on opis/closure (PHP 8 isn't ready, requires FFI for it) or riimu/kit-phpencoder (isn't active, extra layer).
• End user will get native syntax for require and $_ENV.

Disadvantages

• Likely not straightforward to implement.
• Requires maintenance.

[samdark]

https://trello.com/c/zxrr0ArI/98-rewrite-composer-config-plugin-to-use-parser-or-tokenizer-instead-of-serializing

[samdark]

@Mirocow will attempt implementing it.

[samdark]

https://gist.github.com/samdark/86f2b9ff01a96892efbbf254eca8482d

[samdark]

Updated gist above based on recent chat discussions.