Enhancement: add more readable parser error snapshots

[wasd243]

Description

This PR adds an optional errors.stderr snapshot for parser tests, providing compiler-style diagnostics with precise source locations.

Developers can optionally add an errors.stderr file to failing test cases to verify human-readable parser diagnostics.

errors.json is still required for parser-error tests. errors.stderr is only intended to improve readability for long test inputs (e.g. full articles used for dogfooding, Unicode tests).

errors.stderr is not generated automatically. To enable it for a test, create an empty errors.stderr file first.

Example output:

error: RuleFailed  
  ┌─ input.ftml:1:12  
  │    
1 │   ##not color  
  │ ╭───────────^  
2 │ │   
3 │ │ ##|no spec##  
  │ ╰^ color  
  
error: NoRulesMatch  
  ┌─ input.ftml:1:1  
  │  
1 │ ##not color  
  │ ^^ fallback  
  
error: EndOfInput  
  ┌─ input.ftml:5:14  
  │  
5 │ ##only color|  
  │              ^ color  
  
error: NoRulesMatch  
  ┌─ input.ftml:5:1  
  │  
5 │ ##only color|  
  │ ^^ fallback

Dependency

This PR adds codespan-reporting as a dev-dependency to render compiler-style diagnostics for errors.stderr.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.25%. Comparing base (782ce11) to head (4de347e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
+ Coverage   71.17%   71.25%   +0.08%     
==========================================
  Files         220      220              
  Lines        6234     6224      -10     
==========================================
- Hits         4437     4435       -2     
+ Misses       1797     1789       -8     

Flag	Coverage Δ		*Carryforward flag
ftml	64.39% <ø> (ø)		Carriedforward from b7747c7

*This pull request uses carry forward flags. Click here to find out more.
see 10 files with indirect coverage changes

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[wasd243]

I noticed this when testing parser errors with longer .ftml documents.

When the input is large, locating errors from errors.json alone might be difficult, such as:

{  
    "token": "right-block",  
    "rule": "fallback",  
    "span": [  
        // very large number
        // very large number
    ],  
    "kind": "no-rules-match"  
}

The compiler-style errors.stderr output makes the source location immediately visible and makes parser failures easier to debug.