Introduce `Parser` interface.

[OmarMGaber]

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Description

Introduce a Parser interface to support different file types such as .txt, .html, .pdf, etc. This makes the engine extensible and ready for multi-format parsing.

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Why

• Currently, the engine only supports parsing plain .txt files.
• Introducing a common interface allows us to plug in support for other formats cleanly.

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What to Add

• Define a Parser interface with a method like Parse(path string) ([]string, error) that extracts indexable results.
• Refactor the existing .txt parsing logic to implement this interface.
• Update the engine to use the interface instead of hardcoding .txt logic.
• Introduce a parser factory that select parser based on file extension or file headers.

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Example

type Parser interface {
    Parse(path string) ([]string, error)
}

// Usage
parser := parserFactory.GetParser("file.html")
words, err := parser.Parse("file.html")