Fix firefox ability to load file, add firefox tests back (#1380)

## Launch Checklist

This PR fixes the problem with firefox not able to load a file when
clicking the open file button.
It does so by removing a very old dependency: `react-file-reader-input`
It uses the "regular" `<label><a/><input type="file" /><label>`
approach.
It adds tests for both firefox and chrome although I'm not sure the
firefox tests will stay...

 - [x] Briefly describe the changes in this PR.
 - [x] Write tests for all new functionality.
 - [x] Add an entry to `CHANGELOG.md` under the `## main` section.

cypress/e2e/maputnik-cypress-helper.ts

@@ -26,6 +26,22 @@ export default class MaputnikCypressHelper {
       this.helper.when.wait(200);
       this.helper.get.elementByTestId(element).realMouseUp();
     },
+    openFileByFixture: (fixture: string, buttonTestId: string, inputTestId: string) => {
+      cy.window().then((win) => {
+        const file = {
+          text: cy.stub().resolves(cy.fixture(fixture).then(JSON.stringify)),
+        };
+        const fileHandle = {
+          getFile: cy.stub().resolves(file),
+        };
+        if (!win.showOpenFilePicker) {
+          this.helper.get.elementByTestId(inputTestId).selectFile("cypress/fixtures/" + fixture, { force: true });
+        } else {
+          cy.stub(win, "showOpenFilePicker").resolves([fileHandle]);
+          this.helper.get.elementByTestId(buttonTestId).click();
+        }
+      });
+    },
     ...this.helper.when,
   };