Force to run Marp script before VS Code preview script

[yhatt]

For keeping extension sustainabillity, Marp for VS Code has not any scroll-sync logic and relies to VS Code's built-in preview script. Scroll sync is working correctly in almost cases, but sometimes the preview pane brings unexpected scroll when updated webview by editing.

When updating preview, VS Code is trying to restore the previous scroll position by calling scrollTo on loading. A timing of execution this script is same as other extensions contributed scripts included Marp. However, they will run in unpredictable order because they are injected through <script async>. If VS Code ran built-in initialization script before Marp's preview script, a calculated scroll position for stabilization would be wrong due to its styles are based on what have not applied Marp specific styles. On the other hand, the built-in scroll stabilization will work to Marp slides perfectly if ran Marp preview script before built-in script. This is reason why user occasionally see to unexpected scroll.

-- #251

This PR takes different approach from #251: Patch MarkdownContentProvider.prototype.getScripts to force running Marp's preview script before other scripts including VS Code preview.

VS Code's Markdown extension has become able to patch because the engine has exposed to env object of markdown-it renderer since VS Code v1.57 (https://github.com/microsoft/vscode/blob/b0ae2854927c19e4de54edd3890ec681f96476e5/extensions/markdown-language-features/src/markdownEngine.ts#L166-L175). It is obviously a hack but removing async attribute from Marp <script> is the most reliable way to prevent jumping scroll.

Fix #248.