Describe the PR

Fixed the sorting logic for negative numeric string in b-table.
Current sorting does not perform the correct sort if data contains- at the beginning of string.
Because when sorting with thenumeric: true option, it is consistent that the negative string is also sorted as if it were negative number.

Seehttps://jsfiddle.net/pudding_dev/506vbgon/3/
If you sort this table ascending order, result should be like [-100, -20, -10, -5, -1, 1, 5].
However, In current version, it's like [-1, -5, -10, -20, -100, 1, 5].

PR checklist

What kind of change does this PR introduce? (check at least one)

• Bugfix (fixes a boo-boo in the code) -fix(...), requires a patch version update
• Feature (adds a new feature to BootstrapVue) -feat(...), requires a minor version update
• Enhancement (augments an existing feature) -feat(...), requires a minor version update
• ARIA accessibility (fixes or improves ARIA accessibility) -fix(...), requires a patch or minor version update
• Documentation update (improves documentation or typo fixes) -chore(docs), requires a patch version update
• Other (please describe)

Does this PR introduce a breaking change? (check one)

• No
• Yes (please describe since breaking changes require a minor version update)

The PR fulfills these requirements:

• It's submitted to thedev branch,not themaster branch
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If new features/enhancement/fixes are added or changed:

• Includes documentation updates
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• New/updated tests are included and passing (required for new features and enhancements)
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• CodeCov for patch has met target (all changes/updates have been tested)
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If adding a new feature, or changing the functionality of an existing feature, the PR's
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• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)