feat: use Eleventy localization features and Eleventy asset processing

[greatislander]

• This pull request has been linted by running npm run lint without errors
• This pull request has been tested by running npm run start and reviewing affected routes
• This pull request has been built by running npm run build without errors
• This isn't a duplicate of an existing pull request

Description

This PR:

1. Adopts first-party Eleventy localization features and eleventy-plugin-i18n-gettext for string translation.
2. Adopts eleventy-plugin-fluid for asset processing, removing Laravel Mix and Stylus support.
3. Updates other dependencies.
4. Simplifies Eleventy configuration to use default _site directory for build output.
5. Prefixes build helper directories in src with an underscore (_filters, _locales, _shortcodes, _transforms, _utils) to match Eleventy conventions (see: https://www.11ty.dev/docs/config/#directory-for-includes, https://www.11ty.dev/docs/config/#directory-for-layouts-(optional)).
6. Moves to separate configurations for linters, and adds lint-staged, rather than running fluid-lint-all.
7. Adds the Netlify Identity widget to the home page for quicker setup with Netlify Identity and Decap CMS.

Steps to test

Basic behaviour

1. Run npm run start
2. Visit site at http://localhost:8080 and test navigation between French and English content.

Expected behavior: Content available in French and English is linked properly.

Adding a language

1. Follow steps here
2. Run npm run start

Expected behavior: Build functions without errors. Content available in French, English, and the newly added language is linked properly. RSS feed, 404 page, and posts pages are available in the newly added language. CMS functions as expected.

Disabling localization for posts

1. Follow steps here.
2. Run npm run start

Expected behavior: Build functions without errors. Content available in French, and English is linked properly. RSS feed is only available in the default language and posts pages in both languages show default language content. CMS functions as expected.

Disabling localization for full site

1. Follow steps here.
2. Run npm run start

Expected behavior: Build functions without errors. Content is only shown in English. RSS feed, posts page and 404 pages are only available in the default language. CMS functions as expected.

Additional information

Not applicable.

Related issues

Not applicable.

[jobara]

Looking good so far. Is it just the docs that we're waiting on, or was there something else you'd like to add as well?

[greatislander]

Looking good so far. Is it just the docs that we're waiting on, or was there something else you'd like to add as well?

I'll move the utility into eleventy-plugin-fluid, get your review on that, then update this PR to depend on that.

[jobara]

In running through the steps for "Disabling Internationalization for the Entire Site", using either method from step 4, the site is generated without navigation.