feat: add property projection to support coordinates provided in EPSG:27700 and EPSG:3857

[jessicamcinchak]

**Per our dev call chat, opening this for review since it's a useful feature for the map component, even though it doesn't fix the example address marker & Ordnance Survey bug described in the attached Trello card.

Previously, we've assumed all coordinate properties (eg longitude, latitude, markerLongitude, markerLatitude) are provided in "EPSG:4326" (aka WGS84 / World Geodetic System).

This PR introduces a projection property that defaults to "EPSG:4326" and additionally accepts "EPSG:27700" (aka OSGB 1936 / British National Grid) or "EPSG:3857" (aka WGS84 / Web Mercator). We assume that all coordinate properties are provided in the same projection.

Coordinates provided in either projection will be translated to "EPSG:3857" to be rendered as a feature per openlayers & web mapping standards.

Example:

<my-map 
    zoom="20" 
    projection="EPSG:27700" 
    longitude="536227"
    latitude="180489" 
    showMarker
    markerLongitude="536227" 
    markerLatitude="180489" 
/>

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[jessicamcinchak]

Need to use X/Y returned by OS to plot address point or fetch features, not Latitude/Longitude

[jessicamcinchak]

also tried this alternate proj4 definition I found, but it put the marker for 233a Rotherhithe St about 1 block south-east (rather than 1 lot west, which is current error)

// https://epsg.io/27700
proj4.defs(
   "EPSG:27700",
   "+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 " +
     "+x_0=400000 +y_0=-100000 +ellps=airy " +
     "+nadgrids=OSTN15_NTv2_OSGBtoETRS.gsb " +
     "+units=m +no_defs +type=crs"
);

[DafyddLlyr]

+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +nadgrids=OSTN15_NTv2_OSGBtoETRS.gsb +units=m +no_defs +type=crs is what's given by EPSG.io - https://epsg.io/27700.proj4

[jessicamcinchak]

Netlify CI is expected to fail here on new commits because I've updated our site's build/deploy settings over here: #61 (unfortuantely it's a global setting, not per branch).

Once that merges and this is rebased, I think it should be able to build again.

[DafyddLlyr]

Looks good!