Switch from `@messageformat/core` to `intl-messageformat`

[andersk]

In a website with a Content-Security-Policy that does not allow unsafe-eval, the @messageformat/core compiler crashes with EvalError, and upstream has no plans to address that; they recommend pre-compiling all functions using a build step, or switching to intl-messageformat.

• Fixes #545.

[codecov-commenter]

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Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 98.17%. Comparing base (fcb0781) to head (079fd7a). :exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #547   +/-   ##
=======================================
  Coverage   98.17%   98.17%           
=======================================
  Files           2        2           
  Lines         548      548           
=======================================
  Hits          538      538           
  Misses         10       10           

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