Add Punjabi Shahmukhi (Arabic script) to Gurmukhi conversion system

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As described in this paper:

• https://www.researchgate.net/publication/220873925_Punjabi_Machine_Transliteration/link/02e7e53aa7f6299c3a000000/download

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There is a website that implements such transliteration:

• https://sangam.learnpunjabi.org

The paper behind this website:

• https://aclanthology.org/W14-5135.pdf