refactored to rely on yaml.Node during parsing

[ndeloof]

The existing compose file loading pipeline uses map[string]any as its intermediate representation, which loses YAML source location information (file, line number). This makes it impossible to produce precise error messages pointing to the offending line in the source file.

It also forces interpolation and path resolution to happen eagerly, with a single global environment, even though include directives can specify per-file env_file and project_directory.

This PR introduces a new loading path to build a ComposeModel backed by raw *yaml.Node trees. Each node is associated with a NodeContext that captures the environment variables, working directory, and source filename from its originating file. All normalization (interpolation, type casting, path resolution) is deferred until Resolve() is called.

This also allows to get rid of mapstructure dependency an rely on UnmarshallYAML function to cover type conversions