Add support for string unescape

[steve-ky]

Checklist

• I have read the contribution guidelines.
• make test passes.
• make lint passes.
• New and changed code is covered by tests.
• Performance improvements include benchmarks.
• Changes match the documented (not just the implemented) behavior of Shopify.

Support double quoted string unescape, single quoted string won't unescape.

[danog]

Mind rebasing pls?

[Copilot]

Pull Request Overview

This PR adds support for string unescaping and updates several parts of the codebase to use the new any type in place of interface{} while also refining number operations and filter definitions.

• Updated parser functions to accept a precompiled token matcher.
• Modified tests and filters to use int64 and any for improved consistency.
• Adjusted string unescaping logic to differentiate between single‐ and double‐quoted strings.

Reviewed Changes

Copilot reviewed 32 out of 32 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
parser/scanner.go	Updated Scan signature to accept tokenMatcher, ensuring callers supply a precompiled regular expression.
parser/parser_test.go & parser/parser.go	Updated Config instantiation to use NewConfig and pointer receivers for consistency.
parser/config.go	Introduced private delims and tokenMatcher fields along with a new Delims method.
liquid_test.go, engine_test.go, engine_examples_test.go	Replaced interface{} with any in test bindings to match new code conventions.
filters/standard_filters.go	Converted numeric filters to handle int64 values and adjusted filter logic for arithmetic operations.
expressions/*	Revised scanner and parser tests to expect int64 numeric values and improved string unescaping in double-quoted literals.
drops.go & drops_test.go	Updated Drop interface method return types to use any instead of interface{}.

[Copilot]

Using math.Inf(int(ia)) in the 'divided_by' filter introduces a mix of integer and floating-point return types. Consider standardizing the return type to float64 for division so that consumers consistently receive one numeric type.

[Copilot]

[nitpick] Adding a comment here to clarify that only double-quoted strings are unescaped—while single-quoted strings are left intact—would improve code clarity for future maintainers.