Add libmgui v2.0.0

[eunos-1128]

This pull request adds a new recipe for the libmgui package, enabling its build and distribution in a standardized way. The recipe includes metadata, build instructions, dependencies, and testing information, making it easier to integrate libmgui into automated packaging and deployment systems.

New recipe creation and packaging:

• Added recipes/libmgui/recipe.yaml to define the libmgui package, including versioning, source URL, and SHA256 checksum for integrity verification.
• Specified build instructions for Linux, including use of cmake, parallel build, and installation steps. Windows builds are explicitly skipped for now.
• Listed build and host dependencies, with conditional requirements for Linux such as gtk4 and libcanberra.
• Defined package tests to ensure expected contents, including presence of mgui/* files and libmgui.so library.
• Provided metadata such as homepage, repository, license, summary, description, and maintainers for improved documentation and discoverability.

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Checklist

• Title of this PR is meaningful: e.g. "Adding my_nifty_package", not "updated meta.yaml".
• License file is packaged (see here for an example).
• Source is from official source.
• Package does not vendor other packages. (If a package uses the source of another package, they should be separate packages or the licenses of all packages need to be packaged).
• If static libraries are linked in, the license of the static library is packaged.
• Package does not ship static libraries. If static libraries are needed, follow CFEP-18.
• Build number is 0.
• A tarball (url) rather than a repo (e.g. git_url) is used in your recipe (see here for more details).
• GitHub users listed in the maintainer section have posted a comment confirming they are willing to be listed there.
• When in trouble, please check our knowledge base documentation before pinging a team.

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipes/libmgui/recipe.yaml) and found it was in an excellent condition.

[github-actions]

Hi! This is the staged-recipes linter and I found some lint.

It looks like some changes were made outside the recipes/ directory. To ensure everything runs smoothly, please make sure that recipes are only added to the recipes/ directory and no other files are changed.

If these changes are intentional (and you aren't submitting a recipe), please add a maintenance label to the PR.

File-specific lints and/or hints:

• .devcontainer/Dockerfile:

  • lints:
    • Do not edit files outside of the recipes/ directory.

• .devcontainer/devcontainer.json:

  • lints:
    • Do not edit files outside of the recipes/ directory.

[github-actions]

Hi! This is the staged-recipes linter and your PR looks excellent! 🚀

[eunos-1128]

@conda-forge/help-c-cpp, ready for review!