Change escaped character behavior in Strings

[Genarito]

Motivation

Currently, in the String specs it says:

For convenience, some popular characters have a compact escape sequence.
Any Unicode character may be escaped with the \uXXXX or \UXXXXXXXX forms. The escape codes must be valid Unicode scalar values.
All other escape sequences not listed above will be interpreted as literal.

When any escape sequences not listed are used, It'll be more convenient to follow JSON and YAML behavior of raising an error to prevent unexpected problems.

Proposal

Add an InvalidEscapedCharacterError to the specs. Implementations must raise it in case any character not listed in the escape sequence list is used as an escaped character by the user.

Considerations

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Version

Gura 2.0.0 as It adds breaking changes to the existing API. This change would be introduced with Literal Keys (#10) to prevent a huge amount of versions.

Changes

1. Add InvalidEscapedCharacterError definition to the String section.
2. Include changes in a new Changelog section.