Document webpack change: add exportsFields and conditionNames options

[webpack-bot]

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

See pull request: webpack/webpack#10953

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What kind of change does this PR introduce?
closes #9509
exports field feature support

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?

Webpack now supports exports field entrypoint.

Regarding to conditional exports webpack core supports the following conditions:

• "import" - matched when the package is loaded via import or import()
• "require" - matched when the package is loaded via require()
• "node" - matched when options.target is node or electron environment
• "electron" - matched when options.target is electron environment
• "browser" - matched when options.target is web environment
• "worker" - matched when options.target === "webworker"
• "development" - matched when options.mode === "development"
• "webpack"
• "default"

Added some parameters to options.resolve option:

• options.resolve.conditionNames a list of exports field condition names, default list described above in "conditional exports" section
• options.resolve.exportsFields a list of exports fields in description files, default is ["exports"]
• options.resolve.byDependency extra resolve options per dependency category. Typical categories are "commonjs", "amd", "esm".
  This option allows to define specific resolving to each dependency category, e.g. disallow extensions for esm:

module.exports = {
   resolve: {
        byDependency: {
              "esm": {
                     "extensions": []
              }
        }
   }
};

[sokra]

I started on some documentation here: https://gist.github.com/sokra/e032a0f17c1721c71cfced6f14516c62

[benmccann]

The exportsFields documentation is extremely lacking. Questions I have include:

• What is the default value for this field?
• Will the value I supply override the default or be merged with the default?
• If the conditions I supply are merged with the default, will the new values will take precedence over the default values? What's the answer to this question for mainFields?
• If I specify something like import here will that cause it to always be applied or to be applied conditionally only when import is being used?
• If the conditions I supply override the default, an example would be helpful because I guess I would need to check process.env.NODE_ENV to set production or development appropriately and that would be easy to overlook from the current docs