BasicFormat should automatically determine indentation level of a file

### Description

We could take a pretty good guess on the indentation of the file.

There's a few ways we could do this. One would be to grab the indentation up to content in each line, then divide that by common indents and pick the closest match. This is probably good enough, but we can always get more accurate by eg. skipping multiline strings, or rather than an average, instead look at how many lines fit that indentation.

Start by locating the BasicFormat entry point and reading how indentation is currently selected. Compare the proposed approaches for estimating indentation from line content, including how multiline strings would affect the result. Done means BasicFormat can infer a file's indentation reliably, with tests covering representative indentation patterns.

Written by the indexing model from the issue text.