Allow FileSystemProviders to set the read-only file message

[alexr00]

In the GitHub Pull Request and Issues extension, we show PR diffs using a FileSystemProvider and make them read-only when the PR is not checked out. This makes sense since the file is not available on the local system, so there's nothing there to edit.

However, users want to edit these files and are confused when they get the "Cannot edit in read-only editor" message when they start typing (microsoft/vscode-pull-request-github#3450)

I'd like to encourage users to check out the PR when they try to edit these read-only files. One option to do this would be to change the "Cannot edit in read-only editor" message to "Checkout the pull request to edit this file", or even better to have a markdown popup that shows so I can actually give them a link to check out the PR.

To that end, maybe isReadonly, could take a string or, even better, a markdown string:

vscode/src/vscode-dts/vscode.d.ts

Lines 12613 to 12614 in 94ee5f5

	export function registerFileSystemProvider(scheme: string, provider: FileSystemProvider, options?: { readonly isCaseSensitive?: boolean; readonly isReadonly?: boolean }): Disposable;

[alexr00]

Proposal ( | MarkdownString):

vscode/src/vscode-dts/vscode.proposed.readonlyMessage.d.ts

Lines 10 to 13 in dbb7f45

	export namespace workspace {
	export function registerFileSystemProvider(scheme: string, provider: FileSystemProvider, options?: { readonly isCaseSensitive?: boolean; readonly isReadonly?: boolean | MarkdownString }): Disposable;
	}