chore(deps): update dependency babel-plugin-istanbul to v6

This MR contains the following updates:

Package	Type	Update	Change
babel-plugin-istanbul	devDependencies	major	^5.2.0 -> ^6.0.0

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Release Notes

istanbuljs/babel-plugin-istanbul (babel-plugin-istanbul)

v6.1.1

Compare Source

v6.0.0

Compare Source

BREAKING CHANGES

• Drop node.js 6 (#​226)

Features

• Add support for instrumenter options (#​227) (fe08f5b), closes #​208 #​212
• Drop node.js 6 (#​226) (93db21a), closes #​209

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