Add SvelteKit compatibility

[james-camilleri]

The package source has been converted to use ESM syntax. This has had no impact on the dist code (except a few cosmetic changes which I imagine are due to different Svelte versions handling the compilation).

The change includes:

• migration to ESM syntax
• update of test imports to match new ESM structure
• addition of minor note to README to explain SvelteKit compatibility issues

Code has been linted, and the tests all run successfully. I've also packaged the module and tested it on my own project, and it seems to work well with the caveat I've mentioned in the README.

[antony]

Hi @james-camilleri - thank you!

[antony]

Hey - I swapped out the non esm lib - no workarounds required now!

[james-camilleri]

Super, thank you!