Switch relative path to TS 5.5 ${configDir}

[karlhorky]

Closes #394

TypeScript 5.5 introduced ${configDir}, for specifying the directory where the config is:

• https://devblogs.microsoft.com/typescript/announcing-typescript-5-5-beta/#the-configdir-template-variable-for-configuration-files
• Add {root} helper for tsconfig.json (for avoiding multiple ../../) microsoft/TypeScript#37227
• tsconfig.json: Specify <root> in "include" and "exclude" for extends configs microsoft/TypeScript#51213
• Add template variable ${configDir} (name to be determined) for file path substitution microsoft/TypeScript#57485
• Add template variable ${configDir} for substitution of config files directory path microsoft/TypeScript#58042

We can use this to avoid the brittle relative paths in the tsconfig.base.json file