Twenty Seventeen: Add specific font stack for Thai language

Improves legability and design for users viewing the theme with Thai language.

Props imnok, netweb, melchoyce.

Fixes #38937.

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git-svn-id: https://core.svn.wordpress.org/trunk@39422 1a063a9b-81f0-0310-95a4-ce76da25c4cd