Add string representation of syntax paths

[Copilot]

• Add syntax-path->string function to convert syntax paths to string representation
• Add string->syntax-path function to parse strings back to syntax paths
• Add comprehensive tests for both conversion functions
• Export new functions from the module
• Run existing tests to ensure no regressions
• Address code review feedback (remove redundant type check, fix terminology)
• Simplify syntax-path->string using #:before-first parameter
• Enhance error tests with detailed error message assertions
• Update contract to promise immutable-string? return type

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add string representation of syntax paths</issue_title>
<issue_description>Syntax paths that don't contain hash keys are serializable as plain text, unlike syntax objects themselves. This can be useful in a variety of ways: storing paths in databases, writing paths to the console, showing paths in GUIs, etc. A canonical string form of syntax paths seems like a useful thing to implement. Here's a proposed syntax:

• / for the empty syntax path, which represents the root of a syntax object.
• /0 for (syntax-path (list 0)) (the leftmost subexpression of the root)
• /0/1/2 for (syntax-path (list 0 1 2))

The conversion functions should be named syntax-path->string and string->syntax-path. Syntax path strings shouldn't be allowed to end in / except for the root syntax path.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add string representation of syntax paths #653

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[jackfirth]

@copilot

[jackfirth]

@copilot

[coveralls]

Pull Request Test Coverage Report for Build #109

Details

• 33 of 33 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 92.903%

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Totals
Change from base Build #107:	0.02%
Covered Lines:	14412
Relevant Lines:	15513

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💛 - Coveralls