Svelte preprocess

[ljani]

This PR adds support for Svelte's preprocessing functionality, which is required to use ttag-cli with Svelte if the blocks are written in TypeScript.

The PR seems to be functional. However, I'm having some problems writing tests for this and I'd like to have some guidance on the following points:

• I had to convert the TransformFn to async, because svelte.preprocess is async-only. I also changed eg. Babel calls to async. Is this desirable or should sync versions be preferred?
• To load an ESM module in CJS, I need to use dynamic imports. However, because the current setup will transpile any import() calls to require() calls, I need to do some eval hackery to get around this. Is this acceptable or do you have any other suggestions? Should we try to sanitize the (external) input somehow? I don't think I'd like to reconfigure the whole pipeline in this PR. See also Add flag to not transpile dynamic import() when module is CommonJS microsoft/TypeScript#43329.
• The test setup requires a svelte.config.js in the current working directory, or above it, and I'm not sure how to set it up and thus the test is currently failing (in addition to missing the snapshot). I can make this location configurable, but are there any preferences how should I pass the configuration variable? Running from eg. tests/fixtures/testSvelte seems to be okay, updated the PR.
• I'd need to add yet another dependency svelte-preprocess to test preprocessing TypeScript. (This affects only devDependencies, so its not a problem, updated the PR.) I think dependencies are going to get out of hand at some point. Maybe vue-sfc-parser and svelte should be dynamically loaded, if available? Thus they could be moved from dependencies to devDependencies at least, if not removed from package.json.

Thanks for @MrOrz for initial work with #94.

Feel free to finish this PR.

[ljani]

If anyone needs it, here's a rudimentary Vite (or Rollup) plugin for .po files to be used with Svelte, based on ttag-po-loader:

import parser from "ttag-cli/dist/src/lib/parser";
import utils from "ttag-cli/dist/src/lib/utils";
import { createFilter } from "vite";

const filter = createFilter(/\.po$/);

export function poPlugin() {
    return {
        name: "po-plugin",
        transform(src, id) {
            if (filter(id)) {
                const poData = parser.parse(src);

                const messages = utils.iterateTranslations(poData.translations);
                const header = messages.next().value;
                delete header.comments;
                for (const msg of messages) {
                    delete msg.comments;
                }

                const value = JSON.stringify(poData)
                    .replace(/\u2028/g, "\\u2028")
                    .replace(/\u2029/g, "\\u2029");

                return {
                    code: `export default ${value};`,
                };
            }
        },
    };
}

I guess I could have used ttag-po-loader and just call it, but oh well.