Add includeDirectories parameter to buildShaderBundleJson

[bdero]

buildShaderBundleJson now accepts an optional includeDirectories list. Those directories are appended to impellerc's #include search path, after the manifest's own directory and the bundled shader_lib. This lets a build hook compile shaders that #include reusable GLSL shipped by another package: resolve that package's shader directory and pass it here.

The impellerc argument construction is extracted into a new public shaderBundleImpellercArguments(...) helper, mirroring the existing collectShaderBundleDependencies, so it can be unit-tested without invoking impellerc.

Files resolved through the extra directories are not auto-declared as build dependencies; a caller declares them via the build output if edits to them should retrigger the build. Once impellerc --depfile is consumed in --shader-bundle mode (#15), those transitive includes would be tracked automatically.

Motivation

flutter_scene's upcoming custom-material (.fmat) preprocessor emits fragment shaders that #include flutter_scene's framework GLSL. That framework lives in the flutter_scene package, not in the consumer's project, so it is not on the consumer's impellerc include path. This parameter lets the consumer's build hook put flutter_scene's shader directory on the path directly, instead of copying the framework files into the build output on every build.

Tests

New unit tests for shaderBundleImpellercArguments: the two default includes are always emitted, extra directories are appended in order after them, and an empty list matches the default behaviour. Existing tests are unchanged.

Backward compatible: the parameter defaults to empty. Bumps the version to 0.4.4.