Charts: Add TimeSeriesForecastChart component

[annacmc]

Proposed changes

This PR introduces the TimeSeriesForecastChart component - a specialized chart for displaying historical data alongside forecasted values with uncertainty bands.

Key Features

• Historical and forecast data visualization: Displays historical data as a solid line and forecast data as a dashed line
• Uncertainty bands: Shaded area representing the forecast confidence interval between lower and upper bounds
• Forecast divider: Vertical dashed line marking the transition from historical to forecast data
• Generic data support: Accepts any data shape via accessor functions
• Compound composition pattern: Supports TimeSeriesForecastChart.SVG and TimeSeriesForecastChart.HTML for custom children
• Full legend prop parity: Supports all standard legend props (orientation, alignment, maxWidth, textOverflow, etc.)
• Responsive design: Auto-resizes with withResponsive HOC
• Animation support: Optional entrance animation respecting prefers-reduced-motion
• Customizable tooltips: Default tooltip with custom renderer support
• i18n support: Default series labels are translatable

Other information

The component follows existing patterns in the charts library:

• Uses useChartRegistration for chart registry (consistent with other charts)
• Implements compound composition using attachSubComponents and useChartChildren
• Integrates with SingleChartContext for state sharing
• Follows BEM CSS naming conventions

Jetpack product changes

No changes to existing Jetpack functionality. This adds a new chart component to the charts library.

Does this pull request change what data or activity we track or use?

no

[github-actions]

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Code Coverage Summary

6 files are newly checked for coverage. Only the first 5 are listed here.

File	Coverage
projects/js-packages/charts/src/charts/time-series-forecast-chart/private/forecast-divider.tsx	0/10 (0.00%) 💔
projects/js-packages/charts/src/charts/time-series-forecast-chart/private/use-forecast-data.ts	0/27 (0.00%) 💔
projects/js-packages/charts/src/charts/time-series-forecast-chart/time-series-forecast-chart.tsx	0/69 (0.00%) 💔
projects/js-packages/charts/src/charts/time-series-forecast-chart/index.ts	0/0 (—%) 🤷

Full summary · PHP report · JS report

_{If appropriate, add one of these labels to override the failing coverage check: Covered by non-unit tests Use to ignore the Code coverage requirement check when E2Es or other non-unit tests cover the code Coverage tests to be added later Use to ignore the Code coverage requirement check when tests will be added in a follow-up PR I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage.}

[annacmc]

Closing this up as it got a bit unweildy and I've split it up into #46844 & #46845