[WIP] tooltip component updates

[broccolini]

WIP tooltip component updates

• Adjust margin & position to avoid separated caret to fix https://github.com/github/github/issues/62486
• Darken tooltip bg color so works better on dark backgrounds, and doesn't show overlap of caret
• Add tooltip align-left and align-right modifiers https://github.com/github/design-systems/issues/123
• Adjust line-height and padding so tooltip size lands on whole pixels (to prevent fuzziness)
• Move tooltip-specific variables into tooltips module
• Add tooltip stories

Before:

After:

[broccolini]

Closing in favor of #404