feat(tauri): export TitleBarStyle for all platforms

[WSH032]

Similar to #14008 (comment), this PR is mainly for the sake of API symmetry.

I can see set_title_bar_style on all platforms, but TitleBarStyle is only visible on macOS.

This feels odd when reading on docs.rs, because unlike AppHandle::set_activation_policy and ActivationPolicy, which appear together only on macOS, TitleBarStyle is not paired in the same way.

Also, I feel that TitleBarStyle should be exported under mod window (instead of the top-level module), similar to how window::ProgressBarState is handled, since it's only used for Window::set_title_bar_style.

[github-actions]

Package Changes Through 74e4245

There are 10 changes which include tauri-cli with minor, @tauri-apps/cli with minor, tauri-utils with minor, tauri-bundler with minor, tauri with minor, tauri-macos-sign with minor, tauri-runtime-wry with minor, @tauri-apps/api with minor, tauri-runtime with minor, tauri-plugin with minor

Planned Package Versions

The following package releases are the planned based on the context of changes in this pull request.

package	current	next
@tauri-apps/api	2.7.0	2.8.0
tauri-utils	2.6.0	2.7.0
tauri-macos-sign	2.1.0	2.2.0
tauri-bundler	2.5.2	2.6.0
tauri-runtime	2.7.1	2.8.0
tauri-runtime-wry	2.7.2	2.8.0
tauri-codegen	2.3.1	2.3.2
tauri-macros	2.3.2	2.3.3
tauri-plugin	2.3.1	2.4.0
tauri-build	2.3.1	2.3.2
tauri	2.7.0	2.8.0
@tauri-apps/cli	2.7.1	2.8.0
tauri-cli	2.7.1	2.8.0

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