Introduce Taghelper

[JimBobSquarePants]

Prerequisites

• [x] I have written a descriptive pull-request title
• [x] I have verified that there are no overlapping pull-requests open
• [x] I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules :cop:.
• [x] I have provided test coverage for my change (where applicable)

Description

Adds a new ImageSharpTaghelper that greatly simplifies URL generation (including automatic generation of HMAC keys). The TagHelper is designed to be inheritable allowing the addition of custom commands.

Example usage (note, the syntax highlighter here does not do the TagHelper justice. The property values are all strong typed)

<img src="sixlabors.imagesharp.web.png"
     width="300"
     height="200"
     imagesharp-width="600"
     imagesharp-height="400"
     imagesharp-rmode="ResizeMode.Pad"
     imagesharp-rcolor="Color.LimeGreen" />

The TagHelper will only intitialize when both the following criteria are met:

1. The TagHelper is explicitly referenced in _ViewImports.cshtml via @addTagHelper *, SixLabors.ImageSharp.Web
2. The src and imagesharp- prefixed HTML attributes are declared.

[codecov[bot]]

Codecov Report

Merging #290 (e057f93) into main (35e1ae2) will increase coverage by 0%. The diff coverage is 87%.

@@         Coverage Diff          @@
##           main   #290    +/-   ##
====================================
  Coverage    85%    85%            
====================================
  Files        77     83     +6     
  Lines      2199   2375   +176     
  Branches    313    346    +33     
====================================
+ Hits       1871   2026   +155     
- Misses      235    245    +10     
- Partials     93    104    +11     

Flag	Coverage Δ
unittests	85% <87%> (+<1%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/ImageSharp.Web/Middleware/ImageContext.cs	80% <ø> (ø)
...rc/ImageSharp.Web/RequestAuthorizationUtilities.cs	86% <50%> (ø)
src/ImageSharp.Web/CaseHandlingUriBuilder.cs	88% <71%> (ø)
...rc/ImageSharp.Web/TagHelpers/HmacTokenTagHelper.cs	74% <74%> (ø)
src/ImageSharp.Web/TagHelpers/ImageTagHelper.cs	91% <91%> (ø)
...DependencyInjection/ServiceCollectionExtensions.cs	100% <100%> (ø)
src/ImageSharp.Web/Format.cs	100% <100%> (ø)
src/ImageSharp.Web/FormatCommand.cs	100% <100%> (ø)
src/ImageSharp.Web/HMACUtilities.cs	86% <100%> (ø)
.../ImageSharp.Web/Middleware/ImageSharpMiddleware.cs	81% <100%> (-1%)	:arrow_down:
... and 2 more

... and 1 file with indirect coverage changes

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