Auto-extending - The most flexible way to build

@vlucas Thanks Vance! We have big ambitions. We're building the *next* spreadsheet and there are many things we want to build that aren't possible or that would be a much worse experience as an add-on. This launch is a good example of that – we prompt you to turn on auto-extending when we detect you building a time-series table, when a table is auto-extending things adjacent to the table are moved (not overwritten). This builds on past features like calculated columns which lets you add columns to a query result with spreadsheet formulas. All of these features require learning nothing new, they "upgrade" the experience when you do the things you've always done. This level of detail is impossible without owning the whole thing ourselves.