feat: allow any JS identifier in define, not ASCII-only

[jonaskuske]

Description

The define plugin uses RegExp \b boundaries to make sure that it only replaces standalone identifiers (only _PROD_, not _PROD_USE_DEVTOOLS_). But \b only works with ASCII characters and also not with $, so it doesn't work reliably and worse, there can be false positives leading to broken code:

// { define: { ÜPDATE: '60 * 60 * 24' } } →→ /\bÜPDATE\b/g

const update = ÜPDATE // not replaced, \b boundary is between Ü and P!

const AUTOÜPDATE = update && mode === 'auto' // boundary found between O and Ü!
// replaced, errors:
const AUTO60 * 60 * 24 = update && mode === 'auto'

This PR updates the RegExp to work with any valid JS identifier, so $ and Unicode letters can be included. I also added a couple more tests, both for the newly added $ and Unicode functionality as well as for previously untested functionality:

• shouldn't substitute replacement within other identifiers (_OTHER_CONST_CONTAINING_PRODUCTION_)
• shouldn't substitute replacement in property access (someApiResponse._PRODUCTION_)

fix #5956

Additional context

Because the usage of \b for replacements was mentioned in the docs and therefore specced, this technically is a new feature and not a bugfix.

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[jonaskuske]

Rebased and updated to include the exclusion of (trailing) assignments shipped in 2.8: #5515

[jonaskuske]

Greatly simplified the RegExp and fixed the bug where equality operators following an env var would break the replacement. (e.g. process.env.NODE_ENV === 'production')

Tests pass now, and I think this is ready to be merged?

[patak-dev]

LGTM, let's wait for other approvals. And IMO we should merge this one as part of the 3.0 beta, just to play safe.