Add Size & SizeF structs

[wolfgang-ironsoftware]

Title

Add Size & SizeF structs and modify Rectangle & RectangleF to have constructors that make use of these structs.

Description

Adds the Size & SizeF structs (somewhat simplified) from SixLabors.

Fixes #(issue number)

Doesn't fix any particular issue, but will be useful in IronPrint to specify sizes for printing.

Type of change

Please select the relevant option by placing an 'x' inside the brackets, like this: [x].

• [ ] 🐛 Bug fix (non-breaking change which fixes an issue)
• [x] ✨ New feature (non-breaking change which adds functionality)
• [ ] 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] 🏗️ Internal/structural update (non-breaking change that improves code quality, organization, or performance)
• [x] 📚 This change requires a documentation update
• [ ] 🚀 DevOps build chain modification for release
• [ ] 🤖 DevOps build chain modification for CI

How Has This Been Tested?

Added unit tests from SixLabors.

Checklist:

Please run through the checklist as much as possible and mark the items completed by placing an 'x' inside the brackets, like this: [x].

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] I have successfully run all unit tests on Windows
• [x] I have successfully run all unit tests on Linux

Additional Context

Add any other context, screenshots, or information about the pull request here.

[wolfgang-ironsoftware]

@wolfgang-ironsoftware Could you add implicit cast for another library I saw only SixLabors.ImageSharp? We need System.Drawing, SkiaSharp, and Microsoft.Maui.Graphics.

Good idea! I'll add Monday.

[wolfgang-ironsoftware]

@mee-ironsoftware Added implicit converters for other libraries you mentioned.

Also added to SizeF struct (which didn't include them).

Also added VS studio generated GetHashCode() functions as required by the IEquatable interface.

[wolfgang-ironsoftware]

@mee-ironsoftware Added more operators just a second ago. So have conversion both to and from various types.