Provide non-decoded text to `parse` consumers

[jarrodldavis]

Currently, parse from svelte/compiler decodes HTML entities (such as & or ˙). This causes tools like prettier-plugin-svelte that use the AST to generate source text to write decoded entities back to the template. It would be nice if either this decoding happened later (after the parsing phase) or if an additional, non-decoded property was provided on text nodes.