Support "nearest" joinAsof strategy

[StephLin]

Describe your feature request

Rust and Python versions support nearest asof join strategy, while nodejs version does not yet. It would be great if nodejs version do as well. I'm working on this feature, and following are possible changes and test cases.

Possible changes

1. src/lazy/dataframe.rs

    let strategy = match strategy.as_ref() { 
        "forward" => AsofStrategy::Forward, 
        "backward" => AsofStrategy::Backward,
   +    "nearest" => AsofStrategy::Nearest,
        _ => panic!("expected one of {{'forward', 'backward'}}"), 
    }; 

2. polars/lazy/dataframe.ts

   -     strategy?: "backward" | "forward";
   +     strategy?: "backward" | "forward" | "nearest";

3. polars/dataframe.ts

   -     strategy?: "backward" | "forward";
   +     strategy?: "backward" | "forward" | "nearest";

Test cases

let gdp = pl.DataFrame({
   date: [
     new Date('2016-01-01'),
     new Date('2017-01-01'),
     new Date('2018-01-01'),
     new Date('2019-01-01'),
   ],  // note record date: Jan 1st (sorted!)
   gdp: [4164, 4411, 4566, 4696],
})

let population = pl.DataFrame({
   date: [
     new Date('2016-05-12'),
     new Date('2017-05-12'),
     new Date('2017-10-12'),
     new Date('2018-05-12'),
     new Date('2018-10-12'),
     new Date('2019-05-12'),
   ],  // note record date: May 12th (sorted!)
   "population": [82.19, 82.66, 82.95, 83.12, 83.37, 83.52],
})

population.joinAsof(
   gdp,
   {leftOn:"date", rightOn:"date", strategy:"nearest"}
)

It should output

  shape: (6, 3)
  ┌─────────────────────┬────────────┬────────┐
  │ date                ┆ population ┆ gdp    │
  │ ---                 ┆ ---        ┆ ---    │
  │ datetime[ms]        ┆ f64        ┆ f64    │
  ╞═════════════════════╪════════════╪════════╡
  │ 2016-05-12 00:00:00 ┆ 82.19      ┆ 4164.0 │
  │ 2017-05-12 00:00:00 ┆ 82.66      ┆ 4411.0 │
  │ 2017-10-12 00:00:00 ┆ 82.95      ┆ 4566.0 │
  │ 2018-05-12 00:00:00 ┆ 83.12      ┆ 4566.0 │
  │ 2018-10-12 00:00:00 ┆ 83.37      ┆ 4696.0 │
  │ 2019-05-12 00:00:00 ┆ 83.52      ┆ 4696.0 │
  └─────────────────────┴────────────┴────────┘