Improvements to the interactive prompt in the CLI:

Improvements to the interactive prompt in the CLI:
(1) CLI honors SQLITE_PS1 and SQLITE_PS2 envvars as prompt strings,
(2) CLI prompt strings expand various backslash escapes,
(3) No arbitrary length limits on CLI prompt strings,
(4) New default prompt shows the database filename using the \f
escape,
(5) SQLite core adds the sqlite3_incomplate() API, used by
the \H escape in prompt string rendering,
(6) Enhancements to the ".prompt" dot-command.

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