Create mailers with `jetzig generate mailer <name>`. Mailers define
default values for email fields (e.g. subject, from address, etc.).
Mailers use Zmpl for rendering text/HTML parts.
Send an email from a request with `request.mail()`. Call
`deliver(.background, .{})` on the return value to use the built-in
mail job to send the email asynchronously.
Improve query/HTTP request body param parsing - unescape `+` and `%XX`
characters.
Offload as much work as possible into new `jetzigInit` function provided
by Jetzig's `build.zig`. This means that a new Jetzig project's
`build.zig` is now almost completely vanilla with the exception of two
extra lines:
```zig
const jetzig = @import("jetzig");
```
```zig
try jetzig.jetzigInit(b, exe, .{});
```
Implement `jetzig.http.Request.params()` which parses either a JSON
request body or a query param string into a `jetzig.data.Value`.
Allow configuring params for static site generation - configure an array
of params for each endpoint which is then parsed at build time and a
separate JSON and HTML output is generated for each by invoking the
relevant view function and passing in resource ID/params. Params are
stored in generated `routes.zig` for route matching at run time.
Generate views defined with `request: *jetzig.http.StaticRequest` as
static content into `static/` directory so that exported JSON and HTML
can be rendered direct from disk, skipping runtime rendering.
