Add programmatic context to parsing errors

[byCedric]

From: #15 (comment)

I feel like there will be tooling value in having this as as well { line: <line>, column: <column> } on the error. Maybe before line 80 we could add error.position = this.pos?

Another helpful pattern I like to follow (used in node, which is good precedence I think) is to have distinct .code properties for these errors. If we were tooling around errors, checking err.code === 'EOF_ERROR is much better than err.message.startsWith('unexpected token EOF at').