Release stable v1.0.0

Embed testdata as physical Go code to try to get example_test working

Since using go:embed didn't work on pkg.go.dev, this is another attempt
to get the examples to run on the docs website...

Embed testdata/flatgeobuf/*.fgb in example tests

The purpose of this change is to see if it will enable the example
tests to run on https://pkg.go.dev/github.com/gogama/flatgeobuf.

Before this change, at least, they are failing with errors like the one
shown below. If this change isn't successful, I will most likely revert
it and perhaps resort to some other form of embedding, like directly
putting the FGB bytes in a []byte within `example_test.go`.

```
panic: open testdata/flatgeobuf/empty.fgb: no such file or directory

goroutine 1 [running]:
main.openFile({0x4a7312?, 0x40b33e?})
	/tmp/sandbox2858312298/prog.go:13 +0x3c
main.main()
	/tmp/sandbox2858312298/prog.go:24 +0x3d
```

Remove overly restrictive validation on header feature count in write

For FileWriter, there's no good reason for Header() to enforce an
artificially restrictive feature count. Although you cannot *index*
more than math.MaxInt features, you can *write* more than that many
using the streaming approach with multiple calls to Data().

There's an analogous restriction in FileReader.Header() which could
be relaxed in the following way: no validation in Header, but if you
try to call Index() or IndexSearch(), it blows up; and if you try to
call DataRem() with more than math.MaxInt features remaining, it
blows up.

Cut alpha release