fix: allow unquoted names DuckDB reserves but Snowflake doesn't - #482

Fixes #481.

Unquoted table and column names such as semi, pivot and verbose work in Snowflake but failed with a DuckDB parser error. sqlglot's DuckDB generator already quotes known reserved words, but its list is incomplete (reported as tobymao/sqlglot#8804; 30.25.0 added PIVOT, UNPIVOT, LAMBDA and SUMMARIZE, but not the rest).

• Extend the DuckDB generator's reserved-word set with the reserved and type_function categories from duckdb_keywords(), read once through a short-lived in-memory connection at import. The custom FakesnowDuckDB dialect registers separately and does not modify sqlglot's duckdb dialect.
• Exclude Snowflake's documented always-reserved categories from these additional words, preserving sqlglot's existing quoting rules. This keeps by, check, create, group, ilike, in, is, like, null, qualify and tablesample rejected as unquoted table and column names. The Snowflake list is maintained manually because sqlglot's Snowflake dialect has no reserved-keyword list to reuse.
• Use the dialect for cursor statements, COPY INTO's generated INSERTs, and the DESCRIBE query that expands SELECT * in typed CREATE TABLE AS. Explicitly quote column references synthesized from DESCRIBE results and Parquet field names, so names such as "NULL" and "QUALIFY" retain their values in CTAS and COPY transformations.
• Quote user-written identifiers only during SQL generation, after uppercasing and name resolution. Generated quotes do not change the AST's quoted flags or the case of names supplied in double quotes.

Tests cover the affected words as table and column names, metadata casing, quoted lookup and mixed case, ALTER/DROP, UPDATE/DELETE/MERGE, CTEs and aliases, typed CREATE TABLE AS, COPY INTO, and server mode. Another 22 cases check rejection of the 11 Snowflake-reserved words; four cases preserve quoted reserved column names in CTAS and Parquet COPY transformations.

Some accepted Snowflake syntax still fails earlier in sqlglot's Snowflake parser (reported as tobymao/sqlglot#8805):

• returning and glob as table names or column references
• concurrently as a table name
• select using from t
• create table t (exclude int)
• select lateral from t
• describe table pivot

ALTER TABLE pivot RENAME COLUMN semi TO anti works and is covered by a test.

This does not add Snowflake keyword validation. Context-dependent words remain quoted in all contexts: Snowflake accepts asof, left, right, full and natural as column names and in CREATE TABLE, but requires quotes around these table names in a SELECT's FROM clause. fakesnow also accepts those unquoted SELECTs, consistent with the README's existing caveat about accepting a more liberal dialect. Existing differences for some, minus, increment, regexp and rlike remain outside this change.

Checked against a live Snowflake account (10.38): the #481 examples and new positive test cases, including rename and typed CREATE TABLE AS; error 001003 (42000) for all 22 reserved-name rejection cases; and preserved values for "NULL"/"QUALIFY" in typed CTAS and Parquet COPY transformations (using an internal stage for the live COPY check).

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