Problem:
Checking the extension of a file is done often, e.g. in Nvim's codebase
for differentiating Lua and Vimscript files in the runtime. The current
way to do this in Lua is (1) a Lua pattern match, which has pitfalls
such as not considering filenames starting with a dot, or (2)
fnamemodify() which is both hard to discover and hard to use / read if
not very familiar with the possible modifiers.
vim.fs.ext() returns the file extension including the leading dot of
the extension. Similar to the "file extension" implementation of many
other stdlibs (including fnamemodify(file, ":e")), a leading dot
doesn't indicate the start of the extension. E.g.: the .git folder in a
repository doesn't have the extension .git, but it simply has no
extension, similar to a folder named git or any other filename without
dot(s).
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