This PR elaborates on TypeScript types exported by regular-table in a number of useful ways (and fixes some random debris):

• Introduces discriminated union type for CellMetadata to provide precise type information based on cell type (row_header, column_header, corner, body).
• Adds a type discriminant field to all cell metadata, enabling TypeScript narrowing and better IDE autocomplete.
• Adds global TypeScript declarations for the custom element (JSX intrinsics, document.createElement, document.querySelector).
• Split CellMetadata into specific types: CellMetadataRowHeader, CellMetadataColumnHeader, CellMetadataCorner, and CellMetadataBody.
• Added internal CellMetadataBuilder type for partial metadata during cell construction
• Added FPSRecord type for getDrawFPS() return value, getMeta() ro turen CellMetadata.
• Fixed getMeta() to correctly use table_model.header._fetch_cell for column header lookups (was incorrectly using body._fetch_cell).
• Removed unused _reset_viewport(), _reset_scroll(), and clearWidthStyles() methods.