fix(plugin-webpack): allow port to be configurable

[malept]

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• The changes are appropriately documented (if applicable).

Summarize your changes:

Allows the webpack plugin config entry port to override the default port number of 3000, as that is a common port for various other dev servers.

This should be added to the v6 docs if merged.

ISSUES CLOSED: #691

[MarshallOfSound]

Maybe this should be part of the webpack plugin config as well? 🤔

[malept]

Maybe this should be part of the webpack plugin config as well? 🤔

Yep, that works too. I'll fix.

[malept]

Updated.