perf: Change existing struct to readonly struct

[filzrev]

What's included in this PR

• Change struct model to readonly struct.

This change ensure model is not modified after created.
And It can reduce object copy when passing struct as argument. and defensive copy of struct object.
(Thought It's not measurable perf improvement.)

[KalleOlaviNiemitalo]

It's anyway boxed when it is passed as IFileLinkInfo.

[codecov]

Codecov Report

Patch coverage: 97.05% and no project coverage change.

Comparison is base (2de8644) 77.58% compared to head (26e0969) 77.58%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9119   +/-   ##
=======================================
  Coverage   77.58%   77.58%           
=======================================
  Files         591      591           
  Lines       24567    24567           
=======================================
  Hits        19060    19060           
  Misses       5507     5507           

Files Changed	Coverage Δ
src/Docfx.Common/FileAbstractLayer/PathMapping.cs	75.00% <50.00%> (-8.34%)	⬇️
...ine/TemplateProcessors/TemplateModelTransformer.cs	85.26% <100.00%> (ø)
....Build.TableOfContents/TocDocumentProcessorBase.cs	92.20% <100.00%> (ø)
src/Docfx.Common/FileLinkInfo.cs	94.87% <100.00%> (+2.56%)	⬆️
src/Docfx.Plugins/LinkSourceInfo.cs	100.00% <100.00%> (ø)

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[KalleOlaviNiemitalo]

interface IFileLinkInfo seems to be used only for ICustomHrefGenerator.GenerateHref, for which there are no built-in implementations except in a test. So the boxing can only happen when a third-party plugin is involved.

[filzrev]

There is another FileInfo issue that it contains GroupInfo reference type.
So FileLinkInfo is not considered as immutable object.
And it fails to compare structure equity when GroupInfo property have value.

docfx/test/Docfx.Build.Common.Tests/FileLinkInfoTest.cs

Lines 31 to 33 in d4b18eb

	var result = new FileLinkInfo(fromFileInSource, fromFileInDest, href, context);
	Assert.Equal(result, expected);

It might to be better to change FileLinkInfo to class. And implement IEquatable<T> for FileInfo (and GroupInfo).