Implement missing built-in objects: `.Files`, `.Capabilities`, `.Template`

[codeclown]

The most common built-in objects were implemented in commit 9c878dcd1c00e9323302a7562a50e46c44e0ca17

Implement the remaining objects that are documented in Helm docs: https://helm.sh/docs/chart_template_guide/builtin_objects/

Missing objects at the time of writing:

• .Files
• .Capabilities
• .Template