Provide .chezmoi.script context for templated scripts

[bradenhilton]

Is your feature request related to a problem? Please describe.

#3618 (comment)

Describe the solution you'd like

Provide the state (either the latest entry or a reverse-chronological list of all entries) of the current templated script from the scriptState.

Examples:

{{ .chezmoi.script.runAt }} # if latest-only
{{ (index .chezmoi.script 0).runAt }} # if list

runAt specifically should be compatible with other date/time functions, if possible.

Describe alternatives you've considered

N/A

Additional context

N/A