Improved CSV/TSV editing

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> This is a request for changes in the text editor to improve support for delimited text files. Sorry about the length! A CSV file is "just" a text file, but the semantics of the delimiter make the standard text editor a little cumbersome. The main thing it can do better is align the columns. Other helpful features might be handling multi-line content in a single cell, identifying column numbers, the ability to select whole columns, and so on. Most of this is within reach of the current text editor. There are several CSV extensions available. Some use custom editors which look out of place in vscode and lack many of the cool features of the text editor. So I did a proof-of-concept in the vscode source. Here's an example:  You can see my changes at [the sbj42/csv-mode branch in my fork](https://github.com/sbj42/vscode/tree/sbj42/csv-mode), but keep in mind that I don't know the vscode source very well and I cut some corners in making the proof-of-concept. I considered changing the extension API, but I don't think that will be enough. For instance, you could add API hooks for custom tab-like characters (e.g. so comma could be treated like a tab), along with "tab stops" which are computed from the text model and updated as the buffer changes. But that would only go so far. A better idea is to add built-in CSV support to the editor. ### Minimal features Since this is a complex new feature, there are lots of optional pieces to it. In the spirit of the CONTRIBUTING doc's request for one feature per issue, here are the core changes that I think would need to be implemented. I'll ut other related…