Fix URI format exception in relative path computation

[Smaug123]

Originally observed in FSharp.Analyzers.SDK:

> dotnet fsdocs build --projects
src/FSharp.Analyzers.SDK/FSharp.Analyzers.SDK.fsproj
src/FSharp.Analyzers.SDK.Testing/FSharp.Analyzers.SDK.Testing.fsproj
--properties Configuration=Release --eval --clean --strict
cracking projects...
  skipping project 'FSharp.Analyzers.SDK.fsproj' because an error occurred while cracking it:
System.UriFormatException: Invalid URI: The format of the URI could not be determined.
   at System.Uri.CreateThis(String uri, Boolean dontEscape, UriKind uriKind, UriCreationOptions&
creationOptions)
   at System.Uri..ctor(String uriString)
   at Ionide.ProjInfo.VisualTree.relativePathOf(String fromPath, String toPath) in
/_//src/Ionide.ProjInfo/VisualTree.fs:line 23
   at Ionide.ProjInfo.VisualTree.visualPathVSBehaviour(String projPath, String filePath) in
/_//src/Ionide.ProjInfo/VisualTree.fs:line 33
   at Ionide.ProjInfo.VisualTree.getVisualPath(FSharpOption`1 linkMetadata, FSharpOption`1
fullpathMetadata, String identity, String projPath) in /_//src/Ionide.ProjInfo/VisualTree.fs:line 60
   at Ionide.ProjInfo.VisualTree.getCompileProjectItem(FSharpList`1 projItems, String projPath, String
sourceFile) in /_//src/Ionide.ProjInfo/VisualTree.fs:line 82
   at Ionide.ProjInfo.ProjectLoader.compileItems@752.Invoke(String sourceFile)
   at Microsoft.FSharp.Primitives.Basics.List.map[T,TResult](FSharpFunc`2 mapping, FSharpList`1 x) in
D:\a\_work\1\s\src\FSharp.Core\local.fs:line 245
   at Microsoft.FSharp.Collections.ListModule.Map[T,TResult](FSharpFunc`2 mapping, FSharpList`1 list)
in D:\a\_work\1\s\src\FSharp.Core\list.fs:line 97
   at Ionide.ProjInfo.ProjectLoader.mapToProject(String path, IEnumerable`1 compilerArgs, IEnumerable`1
 p2p, IEnumerable`1 compile, IEnumerable`1 nugetRefs, ProjectSdkInfo sdkInfo, IEnumerable`1 props,
IEnumerable`1 customProps) in /_//src/Ionide.ProjInfo/Library.fs:line 750
   at Ionide.ProjInfo.ProjectLoader.getLoadedProjectInfo(String path, FSharpList`1 customProperties,
LoadedProject project) in /_//src/Ionide.ProjInfo/Library.fs:line 831
   at Ionide.ProjInfo.ProjectLoader.getProjectInfo(String path, FSharpList`1 globalProperties,
BinaryLogGeneration binaryLogs, FSharpList`1 customProperties) in
/_//src/Ionide.ProjInfo/Library.fs:line 845
   at fsdocs.Crack.crackProjectFileAndIncludeTargetFrameworks[a](a _slnDir, FSharpList`1
extraMsbuildProperties, String projectFile) in
/home/runner/work/FSharp.Formatting/FSharp.Formatting/src/fsdocs-tool/ProjectCracker.fs:line 290
   at fsdocs.Crack.crackProjectFile[a](a slnDir, FSharpList`1 extraMsbuildProperties, String file) in
/home/runner/work/FSharp.Formatting/FSharp.Formatting/src/fsdocs-tool/ProjectCracker.fs:line 379
   at fsdocs.Crack.projectInfos@484.Invoke(String p) in
/home/runner/work/FSharp.Formatting/FSharp.Formatting/src/fsdocs-tool/ProjectCracker.fs:line 486
Project cracking failed and --strict is on, exiting

The new test at line 2521 fails while the bug exists.

Path.GetRelativePath is in netstandard2.1; ProjInfo targets net8.0 at minimum.

Tests generated by Claude; if you like, I can cut them down to just the one that demonstrates the bug this PR fixes.