Update dependency core-js to v3.12.0

This MR contains the following updates:

Package	Type	Update	Change
core-js	dependencies	minor	3.11.3 -> 3.12.0

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Release Notes

zloirock/core-js

v3.12.0

Compare Source

• Added well-known symbol Symbol.metadata for decorators stage 2 proposal
• Added well-known symbol Symbol.matcher for pattern matching stage 1 proposal
• Fixed regression of V8 ~ Node 0.12 String(Symbol()) bug, #​933

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