Standardize blank lines to no more than 1 or 2

[tconbeer]

The opinion of some formatters for other languages

• https://prettier.io/docs/en/rationale.html#empty-lines
• https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html#empty-lines

Both leave empty lines from original format, but collapse them to a standard number of lines (ie. 0 empty lines remain 0, 7 or 6 or 1 empty lines all become eg 1 empty line)

Black goes even further and adds empty lines in some places (which I think you could consider doing eg around CTEs for readability)

Originally posted by @nfcampos in #230 (comment)

[tconbeer]

My proposal is:

• No more than 2 consecutive blank lines of depth (0,0). In other words, we'll allow 2 blank lines in between statements (e.g., between a {{ config }} block and select, which seems popular)
• No more than 1 consecutive blank line at any other depth.

(This is similar to Black, where there are 2 spaces between module-level classes/functions and 1 space between anything nested deeper than that. Right now we'll just enforce an upper bound, but in the future we can get more opinionated)