Refactor prefixIds (#1561)

Ref https://github.com/svg/svgo/issues/1499

- migrated to visitor plugin api
- covered with tsdoc
- made the plugin idempotent as requested a few times
  Now even manually running svgo a few times will not duplicate
  prefix in ids and classes
- run each plugin test twice to see which plugin need to run many times
  ideally idempotent plugins will allow to get rid of multipass option in v3

test/plugins/_index.test.js

@@ -30,14 +30,21 @@ describe('plugins tests', function () {
             name,
             params: params ? JSON.parse(params) : {},
           };
-          const result = optimize(original, {
-            path: file,
-            plugins: [plugin],
-            js2svg: { pretty: true },
-          });
-          expect(result.error).not.toEqual(expect.anything());
-          //FIXME: results.data has a '\n' at the end while it should not
-          expect(normalize(result.data)).toEqual(should);
+          let lastResultData = original;
+          // test plugins idempotence
+          const exclude = ['addAttributesToSVGElement', 'convertTransform'];
+          const multipass = exclude.includes(name) ? 1 : 2;
+          for (let i = 0; i < multipass; i += 1) {
+            const result = optimize(lastResultData, {
+              path: file,
+              plugins: [plugin],
+              js2svg: { pretty: true },
+            });
+            lastResultData = result.data;
+            expect(result.error).not.toEqual(expect.anything());
+            //FIXME: results.data has a '\n' at the end while it should not
+            expect(normalize(result.data)).toEqual(should);
+          }
         });
       });
     }

test/plugins/prefixIds.test.js

@@ -0,0 +1,24 @@
+'use strict';
+
+const { optimize } = require('../../lib/svgo.js');
+
+test('should extract prefix from path basename', () => {
+  const svg = `<svg id="my-id"></svg>`;
+  expect(
+    optimize(svg, {
+      plugins: ['prefixIds'],
+    }).data
+  ).toEqual(`<svg id="prefix__my-id"/>`);
+  expect(
+    optimize(svg, {
+      plugins: ['prefixIds'],
+      path: 'input.svg',
+    }).data
+  ).toEqual(`<svg id="input_svg__my-id"/>`);
+  expect(
+    optimize(svg, {
+      plugins: ['prefixIds'],
+      path: 'path/to/input.svg',
+    }).data
+  ).toEqual(`<svg id="input_svg__my-id"/>`);
+});