chore: rename default export webExt

[regseb]

When importing the default export, it's recommended to use the same name as the exported variable (cf. import-x/no-rename-default).

This package exports a variable named webext (lowercase). With the following file:

import webExt from 'web-ext';

webExt.cmd.run("...");

ESLint and import-x/no-rename-default report this error:

Caution: index.js has a default export webext. This imports webext as webExt. Check if you meant to write import webext from 'web-ext' instead.

---

With this pull request, we will be able to use the name webExt (camelCase).

[willdurand]

Looks reasonable, thanks