chore: TypeScript 5.9 upgrade

[johnnyreilly]

First code change in ts-loader for a long time, this was caused due to compilation issues with TS 5.9 and the version of webpack being used in the ts-loader repo.

This is probably the underlying cause in TS 5.9: https://devblogs.microsoft.com/typescript/announcing-typescript-5-9/#notable-behavioral-changes

I think that this lead to changes in the webpack types (certainly the existing types did not compile with TS 5.9 anymore). webpack 5 seems to have been upgraded, and I suspect it's @alexander-akait's pull request: webpack/webpack#19777 that lead to the change which allowed ts-loader to fix the 5.9 related issues.

Upgrading the devDependency to the latest webpack@5 in ts-loader and recompiling surfaced different issues in the ts-loader codebase, likely linked to types getting tighter in webpack. The changes have been catered for in ts-loader, mostly this takes the form of more exhaustive null checks / type narrowing shenanigans.

The comparison testpack had to be regenerated entirely to align with the newer version of webpack 5.

Other changes

This PR also drops testing against 4.x versions of TypeScript.

[alexander-akait]

@johnnyreilly Yeah, webpack is now using strict mode for types, so some types need to be checked on null/undefined and in some places narrowing is required

[johnnyreilly]

Excellent! Thanks!