Add SVG namespace attributes by default

closes #2183

Author: Fil

src/plot.js

@@ -158,6 +158,8 @@ export function plot(options = {}) {
   context.ownerSVGElement = svg;
   context.className = className;
   context.projection = createProjection(options, subdimensions);
+  svg.setAttributeNS("http://www.w3.org/2000/xmlns/", "xmlns", "http://www.w3.org/2000/svg");
+  svg.setAttributeNS("http://www.w3.org/2000/xmlns/", "xmlns:xlink", "http://www.w3.org/1999/xlink");
 
   // Allows e.g. the axis mark to determine faceting lazily.
   context.filterFacets = (data, channels) => {

test/output/aaplBollinger.svg

@@ -1,4 +1,4 @@
-<figure><svg class="plot" fill="currentColor" font-family="system-ui, sans-serif" font-size="10" text-anchor="middle" width="640" height="400" viewBox="0 0 640 400" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+<figure><svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" class="plot" fill="currentColor" font-family="system-ui, sans-serif" font-size="10" text-anchor="middle" width="640" height="400" viewBox="0 0 640 400">
     <style>
       :where(.plot) {
         --plot-background: white;