Rename icu_pattern types to indicate that they can hold multiple numbers of placeholders

[sffc]

@younies had assumed that SinglePlaceholderPattern required exactly one placeholder in the pattern, when in fact it supports zero or one. Given that this is primarily an internal-facing type and that this naming is likely to cause additional confusion in the future, we should probably change the names as follows:

• SinglePlaceholderPattern --> OneOrZeroPlaceholderPattern
• DoublePlaceholderPattern --> TwoOneOrZeroPlaceholderPattern

The third type, MultiNamedPlaceholderPattern, can probably stay as-is.