Add support for importing partial templates

[Olian04]

Syntax

Both mustache and handlebars uses {{> }} for partials. So it would make sense for handles to use the same.

{{>someName some.property.path}}

Suggested API

import gleam/io
import handles
import handles/ctx

pub fn main() {
  let assert Ok(index_template) = handles.prepare("
    {{#each users}}
      {{>hello .}}
    {{/each}}
  ")
  let assert Ok(hello_template) = handles.prepare("Hello {{name}}")
  let assert Ok(string) =
    handles.run(index_template, ctx.Dict([
      ctx.Prop("users", ctx.List([
        ctx.Dict([ ctx.Prop("name", ctx.Str("Knatte")) ]),
        ctx.Dict([ ctx.Prop("name", ctx.Str("Fnatte")) ]),
        ctx.Dict([ ctx.Prop("name", ctx.Str("Tjatte")) ]),
      ])),
    ]), [
      #("hello", hello_template)
    ])

  io.debug(string)
}