[Amp story captions] Implement default, overridable CSS

[processprocess]

Description

1. Implement an optional preset attribute who's value can be "default" or "appear" ("karaoke", "popout" and "rainbow" will be added in follow up [Amp story captions] Preset styles #38093).
2. When preset is defined, render captions in shadow dom and apply the associated style.
3. When preset is not defined or an invalid value, render in light dom (current implementation).

Design doc

Spec for default:

Spec for appear (changing size of specific sections is not feasible):