Seperate linestrings are being combined into one multi-linestring.

When I try to use the package and export the shapefile of multiple linestrings, it automatically combines the linestrings into one single multi-linestring.
```js
const data = {
"type": "FeatureCollection",
"features": [
{
"type": "Feature",
"properties": {"id": 1},
"geometry": {
"coordinates": [
[ 15.732725333712267, 18.51499012275039 ], [ 29.796471779359706, 18.19539060475546 ]
],
"type": "LineString"
}
},
{
"type": "Feature",
"properties": {"id": 2},
"geometry": {
"coordinates": [
[ 16.008079368924513, 13.843031299450175 ], [ 29.25915144646919, 9.310680918508353 ]
],
"type": "LineString"
}
}
]
}
const shpOptions = { outputType: 'blob'};
const blob = await shpwrite.zip( data, shpOptions);
```
The blob above will contain one multi-linestring. However, I expect to have two linestrings.

In the library, I changed the following line

https://github.com/mapbox/shp-write/blob/48a0fe72c09f74f6256ac8d4177af272d01317c3/src/geojson.js#L17

to
```js
geometries: shpType === 'POLYLINE' ? oftype.map(l => [justCoords(l)]) : oftype.map(justCoords),
```
This solved the issue to me, but not sure how it will do to actual multi-linestrings.

Start in src/geojson.js around line 17 and reproduce the issue with the FeatureCollection in the report. Check that separate LineString features remain separate in the exported shapefile while actual MultiLineString geometries are still handled correctly; done means the example produces two line features without regressing multi-line inputs.

Written by the indexing model from the issue text.