feat(modal): support alertdialog aria role

[josefaidt]

fixes #272

Description

Adds alert dialog support for default modal as defined in referenced commit.

• adds alert boolean prop to component and story
• adds modalBodyId reactive statement
• adds alertDialogProps with reactive conditional assignment

let alertDialogProps = {};
$: if (alert) {
  if (passiveModal) {
    alertDialogProps.role = "alert";
  } else if (!passiveModal) {
    alertDialogProps.role = "alertdialog";
    alertDialogProps["aria-describedby"] = modalBodyId;
  }
}

[josefaidt]

@metonym would you prefer the reactive conditional look like this? After posting what I had written I feel this may be more appropriate and readable

let alertDialogProps = {};
$: if (alert) {
  if (passiveModal) {
    alertDialogProps.role = "alert";
  }
  if (!passiveModal) {
    alertDialogProps.role = "alertdialog";
    alertDialogProps["aria-describedby"] = modalBodyId;
  }
}

[metonym]

@josefaidt let's go with the second one

[metonym]

Thanks for taking care of this!