Support spaces in file names

[akohout-hai]

Currently, spaces in file names as mentioned in the Readme do not work due to word splitting in the entrypoint.
With these changes, the spell checker correctly processes files with spaces.

[Copilot]

Pull request overview

This PR implements support for file names containing spaces in the spellcheck GitHub Action. The changes convert the SOURCES_LIST variable from string concatenation to a bash array, which properly preserves spaces in file paths and prevents word splitting issues that would otherwise cause the action to fail when processing files with spaces in their names.

Changes:

• Convert SOURCES_LIST from string to bash array to handle spaces in file names
• Update array append operations to use proper bash array syntax with quoted elements
• Replace string emptiness checks with array length checks

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[jonasbn]

This is approved

@akohout-hai thanks for the contribution and assistance. I will ship the release over the weekend, if not earlier.