Make optimizeImports compatible with Svelte 5

[metonym]

Closes #61

Svelte 5 contains breaking changes. One of them is that svelte/compiler no longer exports the walk function.

carbon-preprocess-svelte uses walk to walk the AST in order to re-write Carbon-related imports at compile time.

The officially recommended approach is to import walk from street-walker directly. However, in my initial testing, I found that the latest major version of estree-walker is ESM-only (see Rich-Harris/estree-walker#26).

My goals for this package include the following:

• Keep any direct dependencies to an absolute minimum.
• Ensure the package is broadly compatible. This means continuing to distribute the package in CJS, as Vite-based set-ups support this format (presumably the predominant bundler given the popularity of SvelteKit etc..).
• Avoid including svelte as a direct dependency. I want to avoid using a specific version of svelte with this package. What I mean by this is that the package should use the parser from the Svelte version the user uses. For example, if a user is running Svelte 3/4, it should use the parse from their exact Svelte version.

As such, I've arrived at the following:

• Use walk from estree-walker@2 (which is still CJS-compatible)

From my testing, this approach works with Svelte 4 and Svelte 5.