Apply :duration property for list transition, fixes #10626

[yukukotani]

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

• Bugfix
• [] Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

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

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• 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)

Other information:
duration property for transition-group component accepts a value with type object, string, or number.
If it's object, you can apply like { enter: 500, leave: 800, move: 300 } in ms. In case of string or number, you can just specify duration like 500 as same as transition component.

[posva]

There are a few syntaxes that should be supported and testcases should also be included, so this is a bit longer than it looks like. You can inspire yourself from #4857

[yukukotani]

@posva Done!