Update dependency core-js to v2.6.12

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
core-js (source)	2.6.4 -> 2.6.12

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Release Notes

zloirock/core-js (core-js)

v2.6.12

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• Added code points / code units explicit feature detection in String#at for preventing breakage code which use obsolete String#at proposal polyfill
• Added OPEN_SOURCE_CONTRIBUTOR detection in postinstall
• Added Drone CI detection in postinstall

v2.6.11

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• Returned usage of node -e in the postinstall scripts for better cross-platform compatibility, #​582
• Improved CI detection in the postinstall script, #​707

v2.6.10

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• Show similar postinstall messages only once per npm i, #​597

v2.6.9

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• Some fixes and improvements of the postinstall script like support npm color config (#​556) or adding support of ADBLOCK env variable

v2.6.8

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• Added a workaround of a strange npx bug on postinstall, #​551

v2.6.7

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• Added one more workaround of alternative not completely correct Symbol polyfills, #​550, #​554

v2.6.6

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• Fixed IE8- non-enumerable properties support in Object.{ assign, entries, values }, #​541
• Fixed support of primitives in Object.getOwnPropertySymbols in Chrome 38 / 39, #​539
• Show a message on postinstall

v2.6.5

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• Fixed buggy String#padStart and String#padEnd mobile Safari implementations, #​414.

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