Language guessing: Allow paste contents to override `files.defaultLanguage`?

Speaking with @sbatten it was griped that the "paste contents into an untitled editor to set the language to the paste source" flow doesn't work when you have `files.defaultLanguage` configured. This is intentional AFAIK but maybe worth revisiting? Relevant code: https://github.com/microsoft/vscode/blob/33c614d11a561bfac4061cdb5f24dc4215c62e9e/src/vs/workbench/browser/parts/editor/textResourceEditor.ts#L165-L220 cc @TylerLeonhardt @bpasero