`$regex` flexibility, simpler options, `Reader.toBson`

### DSL

- Allow using `$regex` on a nullable `String` field (!59)
- Rewrote the representation of options to make them simpler to evolve in the future (!61)

### BSON

- Added `BsonDocumentReader.toBson` and `BsonArrayReader.toBson` (9148b99c, !61)
- Added `BsonDocumentReader.asValue` and `BsonArrayReader.asValue` (19348ff2, !63)
- Added `BsonValueWriter.pipe` (#41, 48267c74, !63)
- Renamed `Bson.read` to `Bson.reader` (f503f49e, !61)

### BSON (Multiplatform)

- Read binary data (876b6fee, !58)
- Read JavaScript (21b5e928, !58)
- Read `undefined` (c233d391, !62)
- Implemented `BsonContext.buildArray` (#48, 37f0b868, !63)
- Implemented `BsonFieldWriter.write` (627ec56e, !63)

### Documentation

- Added a mention to `$in` and `$nin` in `FilterQuery` (a1e04fe8, !57)
- Fixed a broken sentence in `BsonDocumentReader` (34ca574d, !57)
- Added the Multiplatform BSON implementation to the API references (888b31ce, !57)