Enable ImplictUsings and use file-scoped namespaces

[iamcarbon]

This PR is up for feedback on whether we should enable file-scoped namespaces on some of all of the markdig projects.

Pulling the indentation in 4 lines on the tests is a fairly nice improvement for readability -- particularly on a 15" laptop.

[iamcarbon]

@MihaZupan @xoofx Let me know what you think. If we do make this change on the main library -- it would make the change history harder to follow -- but improve the experience working with those files moving forward.

[coveralls]

Pull Request Test Coverage Report for Build 3977316982

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 17977 of 18888 (95.18%) changed or added relevant lines in 306 files are covered.
• 246 unchanged lines in 72 files lost coverage.
• Overall coverage remained the same at 93.292%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Markdig.Tests/Program.cs	0	1	0.0%
src/Markdig.Tests/TestMediaLinks.cs	40	41	97.56%
src/Markdig.Tests/TestYamlFrontMatterExtension.cs	37	38	97.37%
src/Markdig/Extensions/Figures/FigureBlockParser.cs	49	50	98.0%
src/Markdig/Helpers/FastStringWriter.cs	142	143	99.3%
src/Markdig/Helpers/LineReader.cs	31	32	96.88%
src/Markdig/Helpers/StringBuilderExtensions.cs	0	1	0.0%
src/Markdig/MarkdownParserContext.cs	0	1	0.0%
src/Markdig/Parsers/HeadingBlockParser.cs	81	82	98.78%
src/Markdig/Parsers/HtmlBlockParser.cs	235	236	99.58%

Files with Coverage Reduction	New Missed Lines	%
src/Markdig/Extensions/Abbreviations/AbbreviationParser.cs	1	95.6%
src/Markdig/Extensions/Emoji/EmojiInline.cs	1	57.14%
src/Markdig/Extensions/Figures/FigureBlockParser.cs	1	95.7%
src/Markdig/Extensions/Footnotes/FootnoteParser.cs	1	93.07%
src/Markdig/Extensions/GenericAttributes/GenericAttributesExtension.cs	1	96.77%
src/Markdig/Extensions/Globalization/GlobalizationExtension.cs	1	86.88%
src/Markdig/Extensions/Mathematics/MathBlockParser.cs	1	92.11%
src/Markdig/Extensions/MediaLinks/MediaLinkExtension.cs	1	93.02%
src/Markdig/Extensions/TaskLists/HtmlTaskListRenderer.cs	1	60.71%
src/Markdig/Extensions/Yaml/YamlFrontMatterParser.cs	1	91.67%

Totals
Change from base Build 3919526952:	0.0%
Covered Lines:	25916
Relevant Lines:	27171

---

💛 - Coveralls

[xoofx]

@MihaZupan @xoofx Let me know what you think. If we do make this change on the main library -- it would make the change history harder to follow -- but improve the experience working with those files moving forward.

Yeah, please go ahead in applying this to the rest of the library, it should be ok. Thanks!

[MihaZupan]

I like the feature, thanks!
My only concern is the quality of the automatic refactoring - when we did this in YARP it messed up indentation in random places. We should just double-check that doesn't happen before merging.

[iamcarbon]

@MihaZupan Agreed on the caution -- I used the refactoring tools to do this file by file, checking them as I went along to make sure everything was indented correctly. We should be OK!

[iamcarbon]

@xoofx @MihaZupan Ready for review. I also fixed a couple spelling errors in the comments while I was double checking the indentation levels.

[iamcarbon]

Here's a direct link to review ignoring whitespace changes: https://github.com/xoofx/markdig/pull/693/files?diff=unified&w=1

[xoofx]

Thanks a lot!