docs: Replace ☕ with emoji ☕️

[pzrq]

This updates :coffee: to ☕️ (emoji) in the README.md.

As the emoji form, it should then render as an emoji in other places like https://www.npmjs.com/package/mocha

Existing screenshot (MacOS, Firefox 70) for posterity:

FWIW, it is a non-ASCII character, though Google's JavaScript style guide states:

Tip: Never make your code less readable simply out of fear that some programs might not handle non-ASCII characters properly. If that happens, those programs are broken and they must be fixed.

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Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions.

Description of the Change

Alternate Designs

Why should this be in core?

Benefits

Possible Drawbacks

Applicable issues

[jsf-clabot]

All committers have signed the CLA.

[coveralls]

Coverage increased (+0.07%) to 92.808% when pulling 29d708b on pzrq:patch-1 into b9fbd69 on mochajs:master.

[juergba]

@pzrq thank you