Document webpack change: import.meta.webpackContext

[webpack-bot]

A pull request by @vankop was merged and maintainers requested a documentation change.

See pull request: webpack/webpack#15446

---

What kind of change does this PR introduce?

feature
closes #15372

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?
import.meta.webpackContext("directory", { recursive: boolean, regExp: RegExp, mode }) returns same as require.context but only for javascript/auto, javascript/esm

[vankop]

also

add parserOptions.importMetaContext. this option enables/disables import.meta.webpackContext evaluation.