Add missing functionality to context

[GandelXIV]

Quick commit porting the following functions to tera2

• https://docs.rs/tera/latest/tera/struct.Context.html#method.get
• https://docs.rs/tera/latest/tera/struct.Context.html#method.into_json
• https://docs.rs/tera/latest/tera/struct.Context.html#method.contains_key