Remove usingnamespace usage and fixup build script / callconv deprecations

[kcbanner]

As of 0.15.x usingnamespace has been removed: ziglang/zig#20663

[Copilot]

Pull Request Overview

This PR modernizes the zglfw codebase to be compatible with Zig 0.15.x by removing the deprecated usingnamespace feature and updating build script patterns. The changes also address callconv deprecations by updating from .C to .c.

Key changes include:

• Replacing usingnamespace with explicit @export calls for Emscripten compatibility functions
• Updating all callback function calling conventions from .C to .c
• Modernizing build.zig to use newer addLibrary API instead of separate static/shared library functions

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/zglfw.zig	Updates calling conventions, removes usingnamespace, adds getInputMode functions, and refactors Emscripten compatibility code
build.zig	Modernizes library creation using addLibrary API and createModule pattern

Comments suppressed due to low confidence (1)

src/zglfw.zig:1235

• The parameter names use anonymous underscores which makes the function signature unclear. Consider using descriptive parameter names like 'joystick_id' and 'state' to improve code readability.

        fn glfwGetGamepadState(_: i32, _: ?*anyopaque) callconv(.c) i32 {