[phase 1] deprecate react 15, fix lifecycle warnings in 16.9

[hshoff]

Related: #431 (comment)

React 16.9 introduces console warnings in DEV-mode for the following lifecycle methods:

• componentWillMount
• componentWillReceiveProps
• componentWillUpdate

These methods continue to work, but react DEV-mode will pollute consoles with warnings that are not actionable on vx users part. To address this and get vx ready for React 17 there will be 2 phases:

• Phase 1 rename legacy lifecycle methods with UNSAFE_* prefix. This requires react peerDeps of ^16.3. This will remove the console warning pollution. This will also drop support for react 15 for packages that use UNSAFE. This is the short-term fix.
• Phase 2 migrate away from legacy lifecycle methods and remove UNSAFE_*. This is the long-term fix.

Why break these into two phases? There are two distinct problems that need addressing: console warning pollution and legacy lifecycle migration. Addressing the first in phase 1 solves most the most pressing problem of console warnings in dev when using 16.9. The long-term fix has the potential to introduce bugs so I'd like to untangle the two problems.

What does this mean for you? If you use: @vx/text @vx/axis @vx/vx you will need to be on React 16.3 or greater.

Thanks to @dagda1 @boom @denkristoffer for raising the issue with @vx/text. We'll review and follow up on the related open PRs in phase 2 (#431, #486)

💥 Breaking Changes

• [text] peerDep react@^16.3, deprecate react 15
• [text] prefix lifecycle methods with UNSAFE_
• [axis] peerDep react@^16.3, deprecate react 15 due to @vx/text dep
• [demo] use react 16.9
• [demo] prefix lifecycle methods with UNSAFE_

🏠 Internal

• [internal] update deps: coveralls, lint-staged, marked

[williaster]

nice. I don't thiiiink this will conflict with the build refactor either 🤞