Problem: There is currently no way to check if a given path is absolute or convert a relative path to an absolute path through the Lua stdlib. `vim.fs.joinpath` does not work when the path is absolute. There is also currently no way to resolve `C:foo\bar` style paths in Windows.
Solution: Add `vim.fs.abspath`, which allows converting any path to an absolute path. This also allows checking if current path is absolute by doing `vim.fs.abspath(path) == path`. It also has support for `C:foo\bar` style paths in Windows.
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