exposing _webui_get_free_port

[sensiblearts]

I want to select a port dynamically for my custom web server, and I figured that since the code is already in webui.c, just expose it in the headers.

(If this PR is accepted, I have a zig-webui PR with an example app.)

[hassandraga]

Yes, it's a good idea, but it should be a new wrapper webui_get_free_port() of _webui_get_free_port().
All APIs start by _ means are internal use only.

Just copy any API and tweak it for _webui_get_free_port().

Example

[sensiblearts]

I added the wrapper and pushed the changes.

[hassandraga]

The C++ wrapper needs return but that's fine, I will fix it later. Thank you @sensiblearts for your contribution.