Chore(sourcemap): renamed with -src.js.map extension

in order to comply with source map spec
https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit#heading=h.9ppdoan5f016
> Optionally, a source map will have the same name as the generated file but with a “.map” extension.  For example, for “page.js” a source map named “page.js.map” would be generated.
See also: Leaflet/Leaflet#5351

build/build.js

@@ -78,7 +78,7 @@ exports.build = function (callback, metaData, compsBase32, buildName) {
         filenamePart = "leaflet.featuregroup.subgroup" + (buildName ? "-" + buildName : ""),
         pathPart = "dist/",
         srcFilename = filenamePart + "-src.js",
-        mapFilename = filenamePart + "-src.map",
+        mapFilename = filenamePart + "-src.js.map",
         srcPath = pathPart + srcFilename,
         mapPath = pathPart + mapFilename,