fix(gatsby): Prevent minifier from compressing output using ES6+ syntax

[shanekenney]

Fixes #8905
Fixes #7003

The previous configuration allowed terser to take opportunities to compress the output code using ES6+ syntax. This PR configures terser to parse ES8 but prevents any ES6+ optimizations in the output.

from https://github.com/terser-js/terser#output-options

The ecma option will only change the output in direct control of the beautifier. Non-compatible features in the abstract syntax tree will still be output as is. For example: an ecma setting of 5 will not convert ES6+ code to ES5.

[pieh]

Was just testing it locally and seems to be working fine, here's example of how output is changed:

diff public/component---src-pages-index-js-7dbbc3a98b9625b0e3a9.js public-master/component---src-pages-index-js-7dbbc3a98b9625b0e3a9.js
40c40
<             return o.a.createElement(s, Object.assign({ t: t }, e))
---
>             return o.a.createElement(s, Object.assign({ t }, e))

so with changes in this PR it produce correct output and before it didn't and it was breaking IE11 for sure

---edit
Note I formatted output code, so diff is actually readable :) It is still minified of course

[pieh]

Thanks @shanekenney for PR and @simonjoom for raising issue and both of you for providing more context! 👍

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[pieh]

Published as gatsby@2.0.25