feat: Add PositionTracker

[fb55]

I took a stab at implementing a position tracker that works for streams. This is something that's been asked for every once in a while, and it seems easy enough to add.

• Directly integrate into Parser

[coveralls]

Pull Request Test Coverage Report for Build 1158889962

• 34 of 34 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 98.286%

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Totals
Change from base Build 1158875845:	0.06%
Covered Lines:	975
Relevant Lines:	992

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💛 - Coveralls

[fb55]

@thewilkybarkid I haven't made up my mind if I want to have this in the module or not. Curious if the parser providing this directly would be a value-add for you.

My current plan is to provide a getPosition(index) method on the parser, which allows you to get a position like this:

const startPos = parser.getPosition(parser.startIndex);

[thewilkybarkid]

Probably worth not adding it to start with at least (feels a little too specialist).

An aside, but I think this has the same problem that posthtml/posthtml-parser#80 had to work around: needing to be able to backtrack (a bit).