feat: add create template method

[CarolinaMoraes]

Description

This PR implements the method for template creation.
Worth mentioning that: besides HTML, it also supports React, as mentioned in the RFC

To support our current template syntax, and avoid users from ending up with a broken React code, the new.email approach to interpolations will be used (this approach was suggested here)

Which means the code (for someone that will use React) will look something like the following:

Template code

<Heading className="text-[24px] font-bold text-gray-900 mb-[16px]">
   Welcome to Your New Beginning, {props.employeeName}
</Heading>

SDK call in a node environment

        const template = await resend.templates.create({
            name: "Welcome to Lumon",
            subject: "Welcome to Lumon Industries - Your journey begins now",
            react: React.createElement(LumonWelcomeEmail, {
                employeeName: "{{{employeeName}}}",
            }),
            variables: [
                { key: "employeeName", type: "string", fallback_value: "Mark S." },
            ]
        })