chore: add TinyTUI benchmark pressure scenarios

- add benchmark project for rendering text overlay markdown and image-line paths

- document benchmark usage in README

- include benchmark project in solution

benchmark/TinyTUI.Benchmarks/TinyTUI.Benchmarks.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\TinyTUI\TinyTUI.csproj" />
+  </ItemGroup>
+
+</Project>