chore: Add fail-fast logics when --serve option enabled & port is already used

[filzrev]

What included in this PR
Add fail-fast logics when --serve option enabled and specified TCP port is already used.

Background
docfx build commands takes some times to complete.
When combined with --serve options.
Command fails on RunServe::Exec phase.
And need to run the `docfx serve' command separately.

This PR add TCP port availability check before start metadata/build commands.
And fail-fast if port is already used.

Test
I've manually confirmed behaviors with following --hostname patterns

• null/ localhost
  • It's bound to loopback addressed (both 127.0.0.1 and ::1)
• *
  • It's bound to all available IPv4/IPv6 addressed
• IPAddress: (e.g. 127.0.0.1 / 127.0.0.2)
  • I’ve confirmed that docfx can successfully host both sites simultaneously.

[codecov]

Codecov Report

Attention: 19 lines in your changes are missing coverage. Please review.

Comparison is base (fe673ec) 74.31% compared to head (6b16be7) 74.25%.
Report is 23 commits behind head on main.

Files	Patch %	Lines
src/docfx/Models/CommandHelper.cs	0.00%	10 Missing ⚠️
src/docfx/Models/BuildCommand.cs	0.00%	6 Missing ⚠️
src/docfx/Models/DefaultCommand.cs	50.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9690      +/-   ##
==========================================
- Coverage   74.31%   74.25%   -0.07%     
==========================================
  Files         536      536              
  Lines       23189    23214      +25     
  Branches     4056     4064       +8     
==========================================
+ Hits        17234    17237       +3     
- Misses       4853     4874      +21     
- Partials     1102     1103       +1     

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[yufeih]

What if we do something like this?

[filzrev]

What if we do something like this?

Personally, I prefer the `fail-fast approach' to the proposed approach.
For the following reasons.

• If BuildCommand is run twice based on the same docfx.config.
  _site contents are overwritten if fail-fast check not exists.
  And the content hosted on `port:8080' will be modified also.
  In this case. it makes no sense to host contents with alternative port.
• When acceptting user input. It needs additional consideration about non-UserInteractive environment.

[yufeih]

Thank you @filzrev !