Add Wa language

[chai2010]

This adds a section for the Wa language, containing the link to the Ada compiler targeting WebAssembly.

• [x] I've read CONTRIBUTING.md.
• [x] Description explains the issue / use-case resolved, and auto-closes the related issue(s) (https://help.github.com/articles/closing-issues-via-commit-messages/).

[icecream0101]

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