Replace default export with named exports (#1998)

## Launch Checklist

See title,
Also removed some "_" from some file names.
This is a pure refactoring, no logic changes.


 - [x] Briefly describe the changes in this PR.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/components/LayerListGroup.tsx

@@ -1,5 +1,5 @@
 import React from "react";
-import Collapser from "./Collapser";
+import { Collapser } from "./Collapser";
 
 type LayerListGroupProps = {
   title: string
@@ -9,7 +9,7 @@ type LayerListGroupProps = {
   "aria-controls"?: string
 };
 
-export default class LayerListGroup extends React.Component<LayerListGroupProps> {
+export class LayerListGroup extends React.Component<LayerListGroupProps> {
   render() {
     return <li className="maputnik-layer-list-group">
       <div className="maputnik-layer-list-group-header"