Using SinglePlaceholderPattern in relative time

[robertbastian]

Warmup for #5256

[robertbastian]

this isn't really human-readable imho, I'd prefer if Pattern serialized to a pattern string instead.

[sffc]

I would too, but that required adding extra trait functions to PatternBackend whereas serializing a list of PatternItems didn't require adding anything to the trait. I made the judgement when writing icu_pattern that the code cost of supporting nicer human-readable serialization was not worth the benefit.

[sffc]

An easier alternative would be to clean up the serialization so long as it is still a Vec<PatternItem>. So for example, instead of

    [
      {
        "Placeholder": "Singleton"
      },
      {
        "Literal": " yıl önce"
      }
    ]

you could have

    [0, " yıl önce"]

which works if the placeholders are numeric. If the placeholders are strings, maybe something like

    [{ "placeholder": "foo" }, " yıl önce"]

[robertbastian]

0 could serialize to "{0}", and { "placeholder": "foo" } could serialize to "{foo}".

[sffc]

Problem with serializing to "{0}" is that you can't tell if that's a placeholder or the literal string, which means we have to deal with escaping, which is a whole bunch of complexity. We should encode whether the segment is a placeholder or a string in the JSON schema.

[sffc]

Nice

[sffc]

Praise: Big size decrease

[robertbastian]

#5369