Guard base-n target against parent-project collision (#184)

### Problem
When JsRuntimeHost is embedded via FetchContent in a parent project that
already declares its own `base-n` INTERFACE target — e.g.
**BabylonNative**, which uses base-n for its Window/Canvas polyfills —
configuring fails:

```
CMake Error at .../jsruntimehost-src/CMakeLists.txt:150 (add_library):
  add_library cannot create target "base-n" because another target with the
  same name already exists.  The existing target is an interface library
  created in source directory ".../Dependencies".
```

This surfaced now because the merged File / FileReader polyfill (#169)
switched its base64 encoder over to the `base-n` dependency, and
BabylonNative declares `base-n` before it fetches JsRuntimeHost.

### Fix
Guard the `base-n` block with `NOT TARGET base-n` so that when a parent
project already provides the target, JsRuntimeHost adopts it instead of
redeclaring. This is the standard CMake subproject pattern and mirrors
BabylonNative's own `if(NOT TARGET glslang)` guards. Standalone
JsRuntimeHost builds are unaffected (no pre-existing target → block runs
as before).

### Validation
- Standalone `NAPI_JAVASCRIPT_ENGINE=Chakra` configure + `UnitTests`
build: green, File polyfill + base-n link as before.
- Unblocks BabylonJS/BabylonNative#1706 (the consuming PR that re-pins
to merged main).

Co-authored-by: Branimir Karadzic <branimirkaradzic@gmail.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

CMakeLists.txt

@@ -145,7 +145,7 @@ if(JSRUNTIMEHOST_POLYFILL_XMLHTTPREQUEST)
     set_property(TARGET UrlLib PROPERTY FOLDER Dependencies)
 endif()
 
-if(JSRUNTIMEHOST_POLYFILL_FILE)
+if(JSRUNTIMEHOST_POLYFILL_FILE AND NOT TARGET base-n)
     FetchContent_MakeAvailable_With_Message(base-n)
     add_library(base-n INTERFACE)
     target_include_directories(base-n INTERFACE "${base-n_SOURCE_DIR}/include")