Document proj4js vs proj4rs projection implementations

projections.md

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+## Projections defined in proj4rs
+
+```
+[
+    (latlong, longlat),
+    (lcc),
+    (etmerc, utm),
+    (tmerc),
+    (aea, leac),
+    (stere, ups),
+    (sterea),
+    (merc, webmerc),
+    (geocent, cart),
+    (somerc),
+    (laea),
+    (moll, wag4, wag5),
+]
+```
+
+## Projections defined in proj4js
+
+- [+] aea
+- [-] aeqd
+- [-] bonne
+- [-] cass
+- [-] cea
+- [-] eqc
+- [-] eqdc
+- [-] eqearth
+- [-] equi
+- [+] etmerc
+- [-] gauss
+- [+] geocent
+- [-] geos
+- [-] gnom
+- [-] gstmerc
+- [-] krovac
+- [+] laea
+- [+] lcc
+- [+] longlat
+- [+] merc
+- [-] mill
+- [+] moll
+- [-] nzmg
+- [-] omerc
+- [-] ortho
+- [-] poly
+- [-] qsc
+- [-] robin
+- [-] sinu
+- [+] somerc
+- [+] stere
+- [+] sterea
+- [+] tmerc
+- [-] tpers
+- [+] utm
+- [-] vandg
+