Add the integration test suite for the readString API

[nipunayf]

Purpose

As a measure to increase the reliability of the readString API, the integration test suite has been used to ensure the robustness of this API. The PR addresses the following points:

1. The API implementations are refactored to follow a uniform path, making the code more readable.
2. The lexer is modified to support the newline character in Windows.

Fixes ballerina-platform/ballerina-library#6141
Fixes ballerina-platform/ballerina-library#4330
Fixes ballerina-platform/ballerina-library#4325

Checklist

• Linked to an issue
• Updated the changelog
• Added tests
• Updated the spec
• Checked native-image compatibility

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.72%. Comparing base (cd505a8) to head (e10169d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   99.13%   99.72%   +0.59%     
==========================================
  Files          44       44              
  Lines        2185     2170      -15     
  Branches     1365     1357       -8     
==========================================
- Hits         2166     2164       -2     
+ Misses         19        6      -13     

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[NipunaRanasinghe]

LGTM