Fix raw text-block indentation

[ChrisPenner]

Overview

Current parse/print of ''' text literals ends up adding a layer of indentation to the literal on every round-trip.

Solutions considered:

1. (Implemented): When parsing a ''' literal, dedent the literal by the indentation of the opening ''', UNLESS there's non-whitespace characters in the area we're de-denting, in which case leave the literal as-is.

• Pros: All possible text-literals are representable
• Cons: Text is parsed differently depending on its contents.

2. Dedent the entire text literal by the largest common indent of the literal.

• Pros: Is a simple, consistent rule that's mostly intuitive.
• Cons: This makes some text-literals unrepresentable.

Implementation notes

I implemented option 1, which involved:

• Trim whitespace equal to the column number of the opening delimiter, if possible on every line, otherwise don't trim anything.
• Adds a newline within {{ }} delimiters for multi-line blocks, without it they would do strange things to the indentation of code-fence blocks; plus it looks nicer IMO

Interesting/controversial decisions

It may be a bit unintuitive that the behaviour changes depending on the actual text in the literal, but chatted with @runarorama and we think this is the simplest way to go that accomplishes the primary goal of roundtrips working.

Test coverage

Transcripts

[pchiusano]

IIRC the current implementation is that:

1. Newline after the opening quote, and just before closing quote is ignored. This avoids awkwardness of needing to put the closing quote on the same line as the end of the string (I’d like to keep this). Also avoids needing to put the content on the same line as the opening quote (also important). Also in conjunction with 2, it allows the opening quote to hang after an =, with the content indented below (this PR will break this behavior but that seems ok to me).
2. Indentation is stripped up to the first non-whitespace character on the first line.

If you’re doing ascii art where the first line starts in the middle of the line, then this heuristic of 2 doesn’t work and this is where round trip errors are happening. Other blocks work fine I think.

[ChrisPenner]

This block is a smidge strange, but it does round-trip. Rúnar and I tried to improve it without much luck :'(