Numerical string sorting incorrectly handles negative values

[deisner]

Describe the bug

When the item data for a column contains strings that represent numerical values, the sorting behavior is as expected, but only if no negative values are included.

Steps to reproduce the bug

1. Go to https://bootstrap-vue.org/docs/components/table#sorting
2. In the Live data, change the items to this (note that the age values are strings):

items: [
          { isActive: true, age: "20", first_name: 'Dickerson', last_name: 'Macdonald' },
          { isActive: false, age: "100", first_name: 'Larsen', last_name: 'Shaw' },
          { isActive: false, age: "1", first_name: 'Geneva', last_name: 'Wilson' },
          { isActive: true, age: "-1", first_name: 'Linus', last_name: 'Carney' },
          { isActive: true, age: "-100", first_name: 'Sally', last_name: 'Carney' },
          { isActive: true, age: "-20", first_name: 'Schroeder', last_name: 'Carney' },
        ]

3. Click the Age table header twice to sort in descending order:

4. The age values, even though strings, are sorted as numbers -- i.e. this is not a dictionary sort (i.e. "20", "100", "1" for a descending sort). But the negative values are in reversed order from what they ought to be.

Expected behavior

The sorted values should be 100, 20, 1, -1, -20, -100, i.e. in decreasing numerical order.

Versions

Libraries:

• BootstrapVue: 2.19.0
• Bootstrap: 4.5.3
• Vue: 2.6.11

Environment:

• Device: Mac
• OS: macOS Catalina
• Browser: Chrome
• Version: 86.0.4240.198

Additional context

I'm quite happy that the sorting is smart enough to recognize that "100" is a number -- I'd just like it to work correctly with negative numbers, too.