lua+js: implement utils.file_info()

This commit introduces mp.utils.file_info() for querying information
on file paths, implemented for both Lua and Javascript.

The function takes a file path as an argument and returns a Lua table /
JS object upon success. The table/object will contain the values:
mode, size, atime, mtime, ctime and the convenience booleans is_file, is_dir.

On error, the Lua side will return `nil, error` and the Javascript side
will return `undefined` (and mark the last error).

This feature utilizes the already existing cross-platform `mp_stat()`
function.

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