Introduce xcstrings-tool-config.yaml configuration file as an alternative to using build settings in Xcode 16.3+

[liamnichols]

Starting in Xcode 16.3, build settings are no longer available in the environment when running a Build Tool plugin executable. This trick is what XCStrings Tool relied on to configure access level or development language (used for legacy .strings files) to generate the correct code.

https://swift-open-source.slack.com/archives/C04MQHWCQKW/p1740417669770009

Since we can no longer use this trick, we need an alternative approach. This change introduces the concept of the xcstrings-tool-config.{yml,yaml,json} file.

It is a regression in friendliness of the plugin, but at least we have a path forward. When using the Build Tool plugin, this file must be placed within the target source files.

The file must have one of the following names:

• xcstrings-tool-config.yml
• xcstrings-tool-config.yaml
• xcstrings-tool-config.json

The file can be either YAML or JSON and has two optional properties:

1. accessLevel - Either internal, public or package
2. developmentLanguage - The value defined in the Package.swift defaultLocalization or Xcode DEVELOPMENT_LANGUAGE build setting
3. verbose - For logging

In an Xcode project, the file must have target membership for the target that the build plugin is assigned to.

[liamnichols]

cc @Brett-Best.. I had some time to tackle this in the end 😅 Let me know what you think of this approach..

I might need to look further into how we discover the config in Xcode project targets because if you need different access levels in different targets, there isn't a way to achieve that. Only in SPM targets

[liamnichols]

Oh and need to update the docs

[rcasula]

Can we also have a way of specifying a custom bundle? This would be really helpful for fixing this #111 and we can tackle also this #121

Edit: I've opened a PR #126 towards this branch