Refactor spotify

[gfscott]

This PR is a substantial refactor of how the Spotify plugin works under the hood, based on the new architecture I came up with for the TED plugin. The public API is unchanged, and end users should not notice any difference to how it works in their Eleventy apps.

Changes in brief:

• Reduces the number of regular expressions from 2 to 1.
  • Older versions had a regex to find each relevant URL, and another to replace each instance. The new architecture has just one, with a single replace function that runs once per processed file.
  • Maintaining only a single regex is easier and less error-prone.
• Adds a lot more tests. The total number of tests run on the plugin has increased from 151 to 8,400.
• Adds examples of each type of embed (album, track, playlist, etc) to the demo site.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 23ae98b8262a12a24dcfc8ffb57636ced4084273

The changes in this PR will be included in the next version bump.

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[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (3d09039) 100.00% compared to head (23ae98b) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #186 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 9 6 -3 Lines 106 96 -10 ========================================= - Hits 106 96 -10 ``` [see 3 files with indirect coverage changes](https://app.codecov.io/gh/gfscott/eleventy-plugin-embed-everything/pull/186/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Graham+F.+Scott)

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