Single number as HighlightedWord causes error

[SRomanova]

This is a very useful package. Thank you for creating it!
I faced some issues in my work with it and I found work around, but since it might be useful for other users, I decided to document it.
To present overview of evaluation, I need to replace some substrings in a template message and to print them out with additional styling. For this task I use combination of ARB file and this package. All values are loading from API and I may receive null for some of them. Here are two issues I faced in my work:

1. with nullable keys:
   I save nullable variables like this
   final value = data?.toString() ?? '';
   and it doesn't return compile error, but causes an ugly result.
   Fix this with adding it with if statement - if(data != null) template: HighlightedWord(textStyle:...),

2. with single numbers:
   If some of keys is number (like 0) it replaces substrings, witch have their own different key.
   Example: "0's points didn't change. Current number of points is 0. 0 has changed technical seniority to Intern."
   where replaced values are bold and instead of userName: HighlightedWord(textStyle:...), for customizing that value is used score: HighlightedWord(textStyle:...) where score = scoreValue.toString() and scoreValue = 0
   Fix this with with adding empty space in front of the number character like this: score = ' ${scoreValue.toString()}'

Hopefully this information will be useful to others. And again: Thank you for creating this package!

[AdarshaBista]

I was also having issues when the first word of the text was a number and was highlighted. In my case, changing the way the index was parsed in _buildSpan fixed it.

Since the index we are interested in is surrounded by <highlight>, we can be sure that it won't contain any leading or trailing whitespace. So, I added a check before calling tryParse to get the index.

int? index;

// Only parse index if nextToDisplay doesn't have any leading or trailing whitespace
if (nextToDisplay.length == nextToDisplay.trim().length) {
  index = int.tryParse(nextToDisplay);
}

I've not tested this thoroughly, so YMMV.

[crizant]

Another error case related to this issue is when the matching number word is a substring of a longer number:
i.e. text: 607856, word: 78.

[crizant]

This has been fixed by #48, I think we don't have to pin it anymore? @desconexo