[clang-format] Recognize ECMAScript module .mjs as JavaScript

PR43085.

Recognize .mjs files as JavaScript. .mjs is the extension for ECMAScript modules.
A specific extension (and associated content type javascript/esm) is
introduced to differentiate it from CommonJS modules and solve some
interoperability problems.

Differential Revision: https://reviews.llvm.org/D66584
Patch by Fergal Daly

llvm-svn: 369732