Add word Mettle

[renitreni]

Add all the definitions of Mettle in a file named Mettle_noun.json in folder M

Refer CONTRIBUTING.md for more details on how to contribute.

Comment the words you'll be adding if you're are working on this issue to avoid conflicts.
Add reference to this issue i.e Issue # in your pull request description.

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[Divyarma]

Hello, I would like to add the word METTLE