Nearley parser

[ballercat]

Move all initial parsing logic to a formal syntax definition (ebnf) and use nearley parser library to parse syntax.

This work enables extending the underlying syntax of Walt, in the future.

[coveralls]

Pull Request Test Coverage Report for Build 114

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 99.478%

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Totals
Change from base Build 77:	0.3%
Covered Lines:	2003
Relevant Lines:	2003

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💛 - Coveralls