Add netcoredbg debug adapter, dotnet locator, and runnables

[Gambl3r08]

Adds debugging support (#12) via the netcoredbg debug adapter, plus a dotnet
debug locator and test/main runnables.

What this does

• netcoredbg adapter: downloaded from GitHub releases on first use, with a
  JSON schema for launch and attach, and a debugger.scm query for inline
  values while paused.
• dotnet locator: turns a dotnet run task into build-then-debug. It
  finds the assembly with dotnet msbuild -getProperty:TargetPath — no shell
  scripts and no filesystem probing, which I believe solves the csproj
  discovery problem described in Add zed-netcoredbg to csharp extension, add runnables #43.
• Runnables: gutter run buttons for [Fact]/[Theory]/[Test]/
  [TestMethod]/[TestCase] methods, test classes (including xUnit classes,
  detected by their contents since xUnit has no class attribute), and Main.

Deliberate limitations

• The locator does not handle dotnet test: the test assembly is a VSTest-driven
  library, and launching it directly exits without running anything (verified).
  Instead there is a "wait for debugger" task using VSTEST_HOST_DEBUG=1, and
  the user attaches to the printed pid — same conclusion Add zed-netcoredbg to csharp extension, add runnables #43 reached.
• MSBuild failures are reported from stdout as well as stderr, since MSBuild
  writes errors like MSB1003 to stdout with an empty stderr.

Relationship to #43: overlapping goal; this implementation needs no shell
scripting for project discovery. Happy to coordinate or merge efforts.

[IldySilva]

this is a must ,