Migrate stream package to TypeScript

[plouc]

Along with the TypeScript migration, I also made the following changes:

• Replaced tooltipFormat with valueFormat, and add a formattedValue property on each computed datum for potential reuse
• Replaced legendLabel and tooltipLabel with a single label property, I don't think we need 2 different properties for this
• Added support for layers
• Added basic unit tests
• Added support for aria attributes on the SVG container
• Added support for custom tooltip and stack tooltip

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[wyze]

Looks good. Build is erroring because of formatting in the test file.

[plouc]

Looks good. Build is erroring because of formatting in the test file.

@wyze, thx for the review, I fixed the formatting issue and added support for custom tooltips to complete another item from #192.