Add PostgreSQL $N and SQLite :name placeholder syntaxes (#263)

[aleks-f]

Extends the parser to recognise two placeholder forms alongside the existing ?:

• $N (PostgreSQL 1-based explicit numbering, e.g. WHERE a = $1)
• :name (SQLite named, e.g. WHERE a = :user)

The ? path is unchanged at every layer (lexer, parser, AST, result ordering); existing tests under test/prepare_tests.cpp pass untouched.

Implementation:

• New lexer rules for $[0-9]+ and :[A-Za-z][A-Za-z0-9_]* placed before the punctuation catch-all. $ was previously unused; : was in the punctuation class but never referenced by any grammar rule, so the longer :identifier match wins without conflict.
• New bison tokens DOLLAR_PARAM (carries the int N) and NAMED_PARAM (carries the identifier). param_expr now has three alternatives. $0 is rejected via YYERROR at parse time.
• Three ExprType values appended (no ordinal change): kExprParameter for ?, kExprParameterDollar for $N (ival = N, 1-based), and kExprParameterNamed for :name (name = identifier). Distinct enum values rather than one with discriminator fields keep consumer switch statements explicit and round-trip printing straightforward.
• Factory functions Expr::makeDollarParameter and makeNamedParameter. isLiteral returns true for all three. The destructor already frees name so no further changes are needed.
• The top-level input rule's renumber loop only touches kExprParameter; $N keeps its explicit ival, :name keeps its name. SQLParserResult::addParameter still sorts by ival, which now orders $N by explicit position and clusters named placeholders.
• sqlhelper print case extended to handle the two new ExprType values.

Tests:

• Three new cases in test/prepare_tests.cpp covering $N, $N declared out of order (verifying ival-sorted result vector), and :name.
• Three new good queries and two new bad queries ($0, lone $) in test/queries/.
• bison -v reports no new shift/reduce or reduce/reduce conflicts.
• make test passes all three checks (SQL tests, valgrind, grammar).

[aleks-f]

I just noticed bison upgrade also went in. If you want that reverted, let me know.