v1.0.0-rc.5: feat: complete the package as a turnkey WebForms app SDK

A vanilla WebForms csproj that references this package now produces
a full IIS-ready deployment from a single `dotnet publish`, with
no hand-maintained MSBuild glue. The minimal csproj is just the
SDK, the target framework, and the PackageReference.

Layout (new buildTransitive .props):
- Defaults `OutputPath=bin\` and drops the
  `<Configuration>\<TargetFramework>\` appends so build output
  mirrors the publish layout — IIS pointed at the project root
  serves the app without an extra copy step. Consumer-set values
  win.

Content + AdditionalFiles collection (.targets):
- Single `EnableDefaultWebFormsGenItems` switch mirrors the SDK's
  `EnableDefaultItems` / `EnableDefaultContentItems` convention:
  cascades from `$(EnableDefaultItems)`, opt out separately with
  `EnableDefaultWebFormsGenItems=false`.
- Globs the markup corpus (aspx/ascx/master/Global.asax) into both
  AdditionalFiles (for the generator) and Content (for publish —
  files stripped to zero bytes post-publish but must exist for
  BuildManager to probe).
- `WebFormsGenContentExtensions` is the customisable list of
  extensions added to the default Content glob (one `**\*.<ext>`
  pattern each). Default set keeps to genuinely universal WebForms
  surface — service handlers, scripts, styles, images, fonts;
  app-specific configs (NLog, log4net) belong in the consumer csproj
  as a standard `<Content Include="..." />`. Override to replace
  the list or append to extend.
- Bare-filename entries (`web.config`, `PrecompiledApp.config`)
  expressed as `**\<file>` globs so MSBuild evaluates them against
  the file system — a literal Include would push a phantom item
  even when the file is absent, breaking the generator on projects
  without a Global.asax (e.g. an .asmx-only library that doesn't
  reference System.Web) and failing publish-time Copy on any project
  missing one of the literals.
- TWF001 PrecompiledApp.config check now only fires when the
  project actually has markup that would emit .compiled sidecars,
  so a library pulling the package solely for the publish pipeline
  isn't nagged for a config file it has no use for.