implement context sensitive language with lexical feedback

The implementation has no regression risks, has an even smaller footprint
compared to the previous attempt involving a pure lexical approach, is higly
predictable and higly configurable.

To turn a word semi-reserved you only need to edit the "SEMI_RESERVED" parser rule,
it's an inclusive list of all the words that should be matched as T_STRING on specific contexts.
Example:

```
method_modifiers function returns_ref indentifier '(' parameter_list ')' ...
```

instead of:

```
method_modifiers function returns_ref T_STRING '(' parameter_list ')' ...
```

TODO: port ext tokenizer

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