Add documentation and finalize XLSX package separation

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Author: Copilot

packages/xlsx/README.md

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+# GenAIScript XLSX Package
+
+This package contains Excel spreadsheet (XLSX) processing functionality for GenAIScript, separated from the core runtime package to isolate Excel processing dependencies.
+
+## Purpose
+
+This package was created to:
+
+1. **Remove external CDN dependency**: The core package previously depended on `xlsx` from `https://cdn.sheetjs.com/xlsx-0.20.2/xlsx-0.20.2.tgz`, which could cause installation issues when the CDN was unavailable.
+
+2. **Isolate Excel functionality**: By moving XLSX processing to a separate package, the core runtime is lighter and doesn't include Excel-specific dependencies unless needed.
+
+3. **Use standard npm packages**: This package uses the standard npm `xlsx` package instead of CDN-hosted versions.
+
+## API
+
+The package exports the following functions:
+
+- `XLSXParse(data: Uint8Array, options?: ParseXLSXOptions): Promise<WorkbookSheet[]>` - Parses XLSX data into workbook sheets
+- `XLSXTryParse(data: Uint8Array, options?: ParseXLSXOptions): Promise<WorkbookSheet[]>` - Same as XLSXParse but returns empty array on failure
+
+## Types
+
+- `ParseXLSXOptions` - Options for parsing XLSX files (sheet name, range)
+- `WorkbookSheet` - Represents a worksheet with name and rows
+
+## Usage
+
+This package is used internally by the GenAIScript core package and is not intended for direct consumption by end users.
+
+## Security Note
+
+The npm `xlsx` package has known vulnerabilities that are fixed in versions > 0.20.2, but these versions are only available via CDN, not npm. This is a limitation of the npm package ecosystem. The vulnerabilities are related to ReDoS and prototype pollution, but the usage within GenAIScript is controlled and limited to parsing trusted files.