Annotate across line breaks on FF, if there are other elements in between

See comment thread here
https://github.com/doccano/doccano/issues/1863

on Firefox
labeling across lines breaks when there is another element in between:

* In the example I try to label: "very loud.\nThe 🏳️‍🌈 car"
* This does not work on Firefox but does work in Chrome.

As comment thread suggests it happens in the method: TextSelectionHandler.getRange()

Do you think this method works before I create a PR?

```
private getRange(): [number, number] {
const selection = window.getSelection();
// get elements.
const startElement = selection!.anchorNode!.parentNode;
const endElement = selection!.focusNode!.parentNode;

// Get TextLine objects.
// This depends on BaseText.vue component.
// See the component in detail.
const startLine = (
startElement as unknown as { annotatorElement: TextLine }
).annotatorElement;
const endLine = (endElement as unknown as { annotatorElement: TextLine })
.annotatorElement;

let startOffset = 0;
let endOffset = 0;
if(startLine === undefined) {
// alternative method

const altStartElement = (startElement?.firstChild as SVGTextElement);
const altStartLine = (
altStartElement as unknown as { annotatorElement: TextLine }
).annotatorElement;
const altStartLineText = altStartElement.innerHTML;

const selectionTexts = selection!.toString().split(/\r\n|\r|\n/);
const altSelectionAnchorStart = altStartLineText.lastIndexOf(selectionTexts[0]);

startOffset = altStartLine.startOffset + altSelectionAnchorStart;
endOffset = endLine.startOffset + selection!.focusOffset;
} else {
// get offsets.
startOffset = startLine.startOffset + selection!.anchorOffset;
endOffset = endLine.startOffset + selection!.focusOffset;
}

selection?.removeAllRanges();
if (startOffset > endOffset) {
return [endOffset, startOffset];
} else {
return [startOffset, endOffset];
}
}
```