[motoko/eng] Add motoko language

[antoniorodr]

• [X ] I solemnly swear that this is all original content of which I am the original author
• [ X] Pull request title is prepended with [language/lang-code] (example [python/fr] for Python in French or [java] for multiple Java translations)
• [ X] Pull request touches only one file (or a set of logically related files with similar changes made)
• [ X] Content changes are aimed at intermediate to experienced programmers (this is a poor format for explaining fundamental programming concepts)
• [ X] If you've changed any part of the YAML Frontmatter, make sure it is formatted according to CONTRIBUTING.md