Return datetime instead of date in metadata

[Bisaloo]

Fix #37.

This development version can be installed with:

if (!require("remotes")) {
  install.packages("remotes")
}
remotes::install_github("ropensci/lightr@datetime")

Datetime is now returned instead of date in all the parsers that already supported date parsing.

Things to do before merging this into main:

• [ ] Figure out how timezone integrate with this change. In the screenshot received by email, the timezone was explicitly specified but from what I've seen, this was not the case with previous version of SpectraSuite. Unix millitimes will be less of an issue since they should all use UTC for the origin.
• [ ] Similarly, the current version of this code returns date time in the user own timezone, which means users in different timezones would get different output with lr_parse_metadata(), which doesn't seem like a satisfying solution.
• [ ] Edit NEWS.md
• [ ] Generally consider the option of returning something different than a vector in lr_parse_XXX[[2]], to allow having mixed types. This would be a big breaking change and would require bumping the version to 2.0

[Bisaloo]

Tests fail because of my second point: my local computer has a different timezone than GitHub action runners.

[Bisaloo]

anytime is not a good choice here because it can only deal with localtime or UTC but doesn't handle other timezones.