[psteinroe]

Stacked on #987.

The helpers the node emitters share were spread over nodes/mod.rs, node_list.rs, string.rs and a few node files, and node files reached them through super::node_list::..., super::string::... or the re-exports in nodes/mod.rs. This PR gives them one place, nodes/utils/, and one way in, a prelude. It moves code and changes no output: every snapshot stays as it is.

What changed

• lib.rs holds only the public API. The crate's modules are private. config.rs holds FormatConfig and its option enums, IndentStyle and KeywordCase included, and builds the renderer's config in one place (From<&FormatConfig> for RenderConfig). format_statement built it a second time by hand.
• The test harnesses used the emitter and renderer directly to format without comments and without the AST check. They now call format_statement_unchecked, which does the same through the public API. pgls_configuration imports IndentStyle and KeywordCase from the crate root.
• nodes/utils/:
  • lists.rs: the list helpers from node_list.rs. The three implementations of the leading comma (comma lists, lists that break in expanded layout, grouped function arguments) and the fill separator become one emit_list_comma. A fill list still leaves own-line comments to the next item, as before.
  • tokens.rs: keywords, identifiers, quoting and literals from string.rs. string.rs keeps only emit_string.
  • layout.rs: call arguments, clause conditions, parenthesized conditions and the layout break, from nodes/mod.rs.
  • comments.rs: emit_node_with, emit_with_comments_at and emit_comments_before_keyword, from nodes/mod.rs.
  • predicates.rs: the shape checks from nodes/mod.rs, and one needs_parentheses(node, Position) in place of the three functions of that name in a_expr.rs, bool_expr.rs and type_cast.rs. The operator precedence table moves along from a_expr.rs.
• nodes/prelude.rs: every node file imports use super::prelude::*; plus its protobuf types. nodes/mod.rs holds the module declarations, the dispatch and emit_statement.
• One file per parser node, named after the node's field in pg_query.proto: bitstring.rs → bit_string.rs, checkpoint_stmt.rs → check_point_stmt.rs, sql_value_function.rs → sqlvalue_function.rs, and six more. DistinctExpr, NullIfExpr, AlternativeSubPlan, MergeWhenClause, AlterTableCmd and FunctionParameter get their own files.

Not in this PR: the helpers that print a construct shared by several statements and still live in a node file (emit_filter_clause, emit_set_clause_list, emit_grant_list, the JSON clauses, the function options, ...) move to nodes/clauses/ in a later PR of this stack. The JSON node files are split there too, because they share those clauses.

The printer's own needs_quoting* in tokens.rs were meant to be replaced by pgls_catalog::quote_identifier / needs_quotes from #981, which follow scan.l and gram.y. #981 isn't merged yet, so they stay for now. Swapping them is a small follow-up once it is.

Tests

• cargo test -p pgls_pretty_print: no snapshot changes.
• cargo test -p pgls_pretty_print --release --test option_matrix -- --ignored
• cargo test -p pgls_workspace -p pgls_configuration format and cargo test -p pgls_cli format
• cargo clippy --workspace --all-targets -- -D warnings