"fix everything in this file" code action

VS Code is investigating an api for better support for fixing all fixable errors in a file with a single action. This action could also be triggered on save. You can find the current proposal [here](https://github.com/Microsoft/vscode/issues/62110#issuecomment-454182230) ## Motivating use case You change a file's extension from `ts` to `js` and start seeing lots of errors about converting your old jsdocs to real types. Thankfully you've enabled `"editor.codeActionsOnSave": { "source.autoFix": true }` in VS Code and TypeScript has implemented this feature. So you just save the file and all your jsdocs are converted to types (along with fixing other trivial errors) ## Current VS Code proposal The current VS Code [proposal](https://github.com/Microsoft/vscode/issues/62110#issuecomment-454182230) for this requires that extensions return a single source code action marked as `autoFix`. This action would contain all auto-fix edits for the file. We'd handle applying these edits on save. ## Proposal for TS To implement this properly for TS, we likely need a new TS server api that would take a file and return a set of edits that fix all of the trivial errors within that file. Making this a single API call per file allows TS to properly handle overlapping fixes within the file. Candidates for what could be fixed automatically: - Errors with one, well known quick fix, such as `convert jsdocs to types` - Trivial coding errors where the intent of the programmer was clear. I see this feature as being especially helpful for JS -> TS. The initial implementation would only need to support fixes for a small set of diagnostics, which could be expanded in future releases. /cc @DanielRosenwasser, @minestarks, @amcasey