feat(dune_lang): format multi-line strings

[Alizter]

Description

Preserve multiline block-string syntax when formatting Dune files that use language version 3.25 or later. Both escaped (\|) and raw (\>) block lines are retained whenever block syntax can represent the original string or template exactly.

The formatter preserves:

• literal contents and newline semantics, including CRLF and EOF-terminated strings;
• the distinction between raw and escaped continuation lines;
• pform expansion and escaping; and
• formatting idempotence for nested and adjacent block strings.

For language versions before 3.25, or when block syntax cannot preserve the original semantics, the formatter emits an ordinary quoted string or template instead.

Implementation

AST and CST parsing

Block strings are concrete syntax, so the AST continues to represent only their meaning as an ordinary quoted string or quoted template. Previously, AST and CST parsing shared one AST-shaped intermediate tree, with comments represented by synthetic AST nodes. That representation cannot retain per-line block syntax without exposing it in the AST or adding another synthetic encoding. The parser therefore now has direct AST and CST loops: this intentional structural duplication keeps the AST API and hot path unchanged while allowing the CST to retain block metadata without an intermediate tree.

Lexer

The lexer retains block metadata only in CST mode. AST mode continues to emit the existing quoted-string or template tokens, while CST mode emits a block-string token containing each logical line's raw/escaped kind and template parts. Its block state distinguishes semantic newlines from physical continuations and handles escapes, pforms, and CRLF. Separate quoted-string and block-string token/CST constructors make invalid mixed states unrepresentable.

Testing

Expect tests cover parsing, CST-to-AST conversion, semantic round trips, and formatting idempotence. Blackbox tests cover formatter output and existing pform/newline behavior. The baseline behavior tests landed separately in #16201 and #16202.

Related work

Supersedes #10780 and #11581.

[Alizter]

I think I will iterate on this a bit further, I'm still not happy with it myself. But others are welcome to take a look.