Rework `Webview.resourceRoot` api proposal

[mjbvz]

Follow up on #72155

The current Webview.resourceRoot api proposal is problematic for a few reasons:

• It's easy to use the value incorrectly, especially when dealing with windows (or, even more fun, windows networked drives!)
• It exposes too much of the implementation detail to extensions, which means that we don't have enough freedom to change the implementation

This iteration, I'd like to revise this API proposal and be closer an idea brought up in #72155:

export interface Webview {
    /** Convert a file uri to a webview uri */
    toResource(resource: Uri): Thenable<Uri>;

   readonly cspRoot: Thenable<string>;
}

The important bits are:

• Encapsulate all the complexity of going from a file uri to a webview URI inside a function
• Add an explicit cspRoot property instead of trying to shoehorn this into toResource.

[mjbvz]

One confusing case discovered while working on #48403

• If you open a workspace at a localhost unc path on windows and open a md preview from that workspace
• We set a <base href> that looks something like file://localhost/c$/workspace
• This works great for resources served from the workspace, but we also want to serve up resources from extensions stored on the local disk. In the html, these end up having paths that look like: file:///c:/vscode/extensions/githubmarkdown/... .
• However the base url causes these urls to be resolved to file://localhost/c:/vscode/extensions/githubmarkdown (which isn't a valid path)

To workaround this, I had to add this code that adds an explicit localhost authority to resources that do no have an authority in cases where the base has an authority:

vscode/extensions/markdown-language-features/src/util/resources.ts

Line 23 in 4c07744

if (base.authority && !resource.authority) {

I'm not sure if this should be a builtin part of this API? For example:

    toResource(resource: Uri, base?: Uri): Thenable<Uri>;

[mjbvz]

Decided to leave the base part out for now. Closing the proposal for this month as it seems to be working well in my testing