Add unexpectedRule option and feature

[log4b0at]

PR type

• Bug fix (non-breaking change which fixes an issue): no
• New feature (non-breaking change which adds functionality): yes
• Breaking change (fix or feature that would cause existing functionality to change): no
• Documentation change: no

Prerequisites

• I have read the CONTRIBUTING.md document: yes
• I have updated the documentation accordingly: no
• I have added tests to cover my changes: no

Description

A little patch to add the unexpected rule feature. See #607

Usage:

• Add the unexpectedRule option to set the unexpected rule name, like in the following example

PEG.generate(/* source */, {unexpectedRule: 'unexpected'})

• Add the rule in your parser grammar

unexpected =
  Function { error("unexpected function declaration here"); } /
  Identifier { error("unexpected identifier here"); };

Note:
Maybe we should add a correct error message when the unexpectedRule is defined but the rule named isn't present in the grammar.

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