Implement warning for invalid unicode characters

[mikir]

Consider to implement [encoding] warning in case of any invalid unicode characters in the schema. Example:

const string STRING = "Contains \u200B: CONSUMPTION_​SPEED_​DEPENDENCY";

The string above contains invisible unicode '\u200B' after underscode character.

Please, consider as well to improve [encoding] warnings. Currently, these warnings are fired immediatelly during parsing of package not during checking phase. If schema contains a lot of imported packages, one can easily overlooked this warning because it will be scrolled up immediatelly.