Add a newline-sensitive lexer mode independent of indent

[johnsoncodehk]

Closes #10.

Problem

Some DSLs are newline-aware but not indent-aware: statements are line-delimited, but nesting is via delimiters/expressions rather than indentation (e.g. dotenv-style env specs). The only way to get significant newlines was to opt into indent, which also brings INDENT/DEDENT stack management and YAML block-scalar semantics.

Approach

newline is the line-boundary + flow-suspension layer that indent already builds on (indent = newline + indent stack + YAML semantics), so instead of a parallel mode this factors out the shared base. A single lineSensitive gate drives the shared lexer machinery; the line-start routine forks — an indent grammar measures columns and runs the stack, a newline-only grammar emits one NEWLINE token per significant boundary. Declaring both is rejected.

The parser needs no change: INDENT/DEDENT/NEWLINE are ordinary tokens a grammar references as many(Newline, Stmt), with no balanced-pair or stack assumptions. In tree-sitter the NEWLINE token is a stateless external (modeled on the raw_text scanner); extras stays /\s/ because tree-sitter runs the external scanner before skipping extras, so a significant newline is taken at statement boundaries while a flow-internal newline falls back to extras as whitespace.

Changes

• src/types.ts — NewlineConfig + CstGrammar.newline
• src/api.ts — passthrough + indent/newline mutual exclusion
• src/gen-lexer.ts — the layered emission
• src/emit-parser.ts — bake the newline config
• src/gen-treesitter.ts — NEWLINE stateless external scanner
• test/newline-mode.ts — inline env-spec grammar across all four backends + a real tree-sitter generate/parse

Verification

• All seven existing grammars regenerate byte-identically (npm run gen)
• TS parser conformance unchanged vs baseline (5386/5659)
• gate:treesitter 96.0% (>= floor, still beats official)
• agnostic 9/9, sanity 15/15, newline-mode 29/29 (incl. a real tree-sitter parse showing flow-internal newlines are suppressed)