feat(combinators/lookahead): add `lookahead` combinator

[norskeld]

This PR:

• Fixes a bug in string and ustring parsers regarding pos.
• Adds lookahead combinator.

Related to #50

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Name	Status	Preview	Updated
sigma	✅ Ready (Inspect)	Visit Preview	Jan 16, 2023 at 5:14PM (UTC)

[coveralls]

Pull Request Test Coverage Report for Build 3932583682

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 3926631607:	0.0%
Covered Lines:	255
Relevant Lines:	255

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💛 - Coveralls

[norskeld]

:tada: This PR is included in version 3.4.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: