Add `wp config add` and `wp config update` commands

[Copilot]

• [x] Add new config add command that adds a constant/variable (fails if it already exists)
• [x] Add new config update command that updates or adds a constant/variable (upsert behavior)
• [x] Keep config set command for backward compatibility
• [x] Add tests for config add command
• [x] Add tests for config update command
• [x] Update composer.json to register new commands
• [x] Update class docblock with examples for new commands
• [x] Address code review feedback
• [x] Security scan with CodeQL (no issues found)
• [x] Verify implementation
• [x] Address PR feedback:
  • Changed WP_DEBUG to WP_CUSTOM_CONSTANT in config-add test (WP_DEBUG exists by default)
  • Fixed assertion in config-update test to expect "Updated" instead of "Added"
  • Aligned equal signs in Config_Command.php

Original prompt

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This section details on the original issue you should resolve

<issue_title>Abstract config set --add to config add</issue_title> <issue_description>config set --add is a deviation from our standard syntax. Typically, set and add are two separate operations (e.g. option set vs. option add, and cache set vs. cache add).

It'd be better to abstract config set --add to config add.

We may also consider a config update, which would add the constant if it didn't exist, or update it if it did.</issue_description>

<agent_instructions>Add new wp config add & wp config update commands for this logic.

config set should be kept for backward compatibility.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@danielbachhuber @wp-cli/committers Does this seem reasonable to you? @schlessera Hehe, I think I started out with `wp config add` & `wp config update`, but then later changed it.

• Fixes wp-cli/config-command#46

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[codecov[bot]]

Codecov Report

:x: Patch coverage is 97.22222% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/Config_Command.php	97.22%	2 Missing :warning:

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