add mise + eslint to gist-web + upgrade node to 20

[karngyan]

Recently pushed a change here https://github.com/customerio/gist-web/pull/68/files where a method was not imported but used, as vscode din't complain.

Changes

• Adds a basic mise config for node version.
• Upgrade node to 20 for builds. (Gonna do a round of testing across cdp and sdk before merging)
• This adds a minimal eslint config, which would only add warnings in the editor. No pre commit hooks or build blockers yet.
• A future PR would have all the eslint warnings resolved. But I don't wanna do that without end to end manual testing of the generated script.

Current errors:

• Planning on a future set of changes with prettier, typescript-ifying this repo, and also adding automated tests so folks can make changes to it confidently.

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Now at least devs would see errors like:

[stephen-pope-customer-io]

This is the main portion of https://linear.app/customerio/issue/INAPP-12846/add-linter-to-gist-web