core: Add pathexpand interpolation function

[grubernaut]

Adds the pathexpand interpolation function to allow users to expand ~ to the home directory in filepath strings.

Fixes: #11268

[stack72]

LGTM!

[apparentlymart]

Should there be a warning in the docs that using this will likely make your config detect spurious diffs when run by multiple users or on multiple hosts? Fine to use this with file(...) but possibly less fine to use it directly in any resource argument.

(People already get confused by the similar problem caused by path.module and ask about it somewhat frequently.)

[grubernaut]

@apparentlymart good point, yeah I can add some documentation to state that.

[ghost]

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