Updated generator to check for expired context

[Aerek-Yasa]

The generator does not currently check if the context has expired. This pull request updates the generator so that it generators a check for an expired context. Many of the variables in the generator have names such as templ_7745c5c3_W. It was not known what naming convention was used for this, so the variable name ctxErr was used instead.

Here is a minimal example with the current output and the expected output.

// component.templ
package main

templ Component() {
    
}

// main.go
package main

import (
	"context"
	"fmt"
	"io"
	"time"
)

func main() {
	comp := Component()
	ctx, cancel := context.WithTimeout(context.Background(), time.Millisecond*200)
	defer cancel()
	time.Sleep(1 * time.Second)
	err := comp.Render(ctx, io.Discard)
	if err != nil {
		fmt.Println(err)
		return
	} else {
		fmt.Println("No error!")
	}
}

Expected output: context deadline exceeded.
Current output: No error!

[joerdav]

It could be worth an additional test, just to ensure that a cancelled context being passed to a component means it doesn't get rendered, and an error is returned. And then I think this is good to go from my perspective. Unless we think this behaviour warrants some documentation, I suppose it is possible that partial html responses could be returned if a request is cancelled.

[Aerek-Yasa]

A test was added that fails if Render() does not return an error when called with an expired context.Context.
Documentation was added that specifies partial output is possible when Render() fails. A partial rendering does not violate the general Go convention that other output parameters can not be trusted to be anything specific when the error output parameter is not nil.

~Aerek

[a-h]

Thanks a lot for the change. Nice one!