fix(splitter): keep CTE queries attached to inserts

[edjubert]

What kind of change does this PR introduce?

Bug fix.

What is the current behavior?

A valid INSERT INTO … WITH … SELECT statement is split after the target column
list because the statement splitter treats WITH as the start of a new
statement.

PostgreSQL then receives the incomplete INSERT INTO … (...) fragment and
reports syntax error at end of input.

What is the new behavior?

WITH is now handled like SELECT while splitting an INSERT, so a
CTE-backed insert remains a single statement until its terminating semicolon.

A regression test also verifies that a subsequent semicolon-terminated
statement is still split separately.

Additional context

Validated with:

cargo test -p pgls_statement_splitter
cargo fmt --all -- --check
cargo clippy --all-targets --all-features -- -D warnings