Add the ReportCallback type back but deprecate it

[philipwalton]

Fixes #482 by re-adding the ReportCallback type but marking it as deprecated.

In most cases developers should not need to explicitly type their metric function callbacks (e.g. the type can be inferred from usage), but in the event that you do, the recommended replacement for ReportCallback is either:

• Specify a metric-specific function type, e.g. (meric: LCPMetric) => void, or
• Specify a type that could be used for any metric function, e.g. (metric: MetricType) => void

The MetricType type can be imported as follows:

// If using the "standard" build:
import type {MetricType} from 'web-vitals';

// If using the "attribution" build:
import type {MetricType} from 'web-vitals/attribution';

[tunetheweb]

LGTM with one suggestion.

[brendankenny]

LGTM!

Deprecation text looks good in vscode: