fix: add default value for optional fields

[zshipko]

Closes #10

Currently when constructing a type with nullable fields the user is required to explicitly pass None if no argument is needed.

This sets nullable fields to None by default - to do this we need to list all non-nullable fields first, then the nullable ones because Python requires arguments with default values to be listed last. This means the order of arguments for type constructors might be in a different order from the schema and other PDKs, but I'm not sure there is another way around this.

[bhelx]

If the only difference b/w these two loops is the nullable part. maybe we can merge them something like this?

@dataclass
class <%- capitalize(schema.name) %>(extism.Json):
<% schema.properties.forEach(p => { -%>
<% if (p.description) { -%>
  # <%- formatCommentBlock(p.description, "# ") %>
<% } -%>
<% if (p.nullable) {%>
  <%- p.name %>: <%- toPythonType(p) %> = None
<% } else { %>
  <%- p.name %>: <%- toPythonType(p) %>
<% } %>
<% }) %>

<% } %>
<% }); %>

or just customize the = None part

[zshipko]

It's an ordering issue, the fields with default values have to come after the fields with no defaults.