Reorganize MathEditorSwift and MathKeyboardSwiftUI into standard SPM layout

[maitbayev]

Motivation

• Standardize package layout so SwiftPM can discover targets and tests using the default conventions (Sources/<TargetName> and Tests/<TargetName>).
• Remove custom path overrides in Package.swift to simplify the manifest and reduce maintenance when moving files.
• Ensure resources bundled with MathKeyboardSwiftUI are colocated under the target directory so .process("Resources") works with the standard layout.

Description

• Moved MathEditorSwift sources from MathEditorSwift/Sources/MathEditorSwift to Sources/MathEditorSwift and tests from MathEditorSwift/Tests/MathEditorSwiftTests to Tests/MathEditorSwiftTests.
• Moved MathKeyboardSwiftUI sources and resource assets from MathKeyboardSwiftUI/Sources/MathKeyboardSwiftUI to Sources/MathKeyboardSwiftUI (many asset catalogs and image files relocated accordingly).
• Updated Package.swift to remove explicit path overrides for the MathEditorSwift, MathEditorSwiftTests, and MathKeyboardSwiftUI targets while preserving declared dependencies and the resources: [.process("Resources")] entry.

Testing

• Ran swift test, which attempted to fetch the external dependency https://github.com/maitbayev/iosMath.git and failed due to a network error (CONNECT tunnel failed, response 403), so unit tests could not complete in this environment.
• Verified the Package.swift manifest and representative source/test files were updated and are located under Sources/ and Tests/ respectively by inspecting file headers and manifest lines (manual manifest/file checks succeeded).

---

Codex Task