Build code from strings more often

[benmccann]

Just as is being done in #556, I think we could simplify a lot of the addon code and make it more readable by building the expression from a string or potentially just returning a string directly instead of building an AST.

eslint.config.js, paraglide/+page.svelte, and the Vite config modification in the vitest addon are a few that looked like they could really be simplified - though I wouldn't be surprised if there were others as well.