The Page TSConfig which is normally stored in a field in the root page can be moved to the file  Configuration/TSConfig/Page.ts .

In order to load this a line needs to be added to  ext_tables.php :

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPageTSConfig(
 '<INCLUDE_TYPOSCRIPT: source="FILE:EXT:' . $_EXTKEY . '/Configuration/TSConfig/Page.ts">'
);

Although the actual TSConfig could be included in  ext_tables.php it's of course easier for maintenance to have it in a separate file.

In a similar way it's possible to load user TSConfig ( addUserTSConfig() ).