Add ParseStmt, ParseExpr and FormatBeautify convenience API

[git-hulk]

Summary

Purely additive public API for the three most common call patterns that previously required boilerplate:

// Parse exactly one statement (errors on zero or multiple statements)
stmt, err := clickhouse.ParseStmt("SELECT a FROM t")

// Parse an expression fragment, e.g. a column expression or function call
expr, err := clickhouse.ParseExpr("toDate(created_at) + 1")

// One-call beautified formatting (counterpart of Format)
sql := clickhouse.FormatBeautify(stmt)

Previously a single statement needed NewParser(sql).ParseStmts() plus slice-length handling, expression fragments could not be parsed at all without wrapping them in a dummy SELECT, and beautified output required NewFormatter().WithBeautify().WriteExpr() wiring.

Changes

• New parser/api.go with ParseStmt, ParseExpr, FormatBeautify; errors flow through the existing wrapError path so they carry positions and caret rendering.
• CLI -beautify path now uses FormatBeautify (behavior unchanged, verified by CLI smoke test).
• README section documenting the helpers.

The DDL.Type() → typed Kind enum item from the same backlog entry is intentionally left out: it is a breaking change and belongs with the v0.5/v1 interface-split batch.

Testing

New api_test.go covering single/zero/multiple statements, fragment parsing, trailing-token rejection, and FormatBeautify equivalence with the long-form formatter plus reparse round-trip. Full suite passes including -race -compatible.

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