Implement the file __close metamethod

[arnodel]

In Lua 5.4 files have a __close metamethod that closes a file when it goes out of scope e.g.:

do
    local f <close> = io.open("hello")
    -- do stuff
end
-- At this point the file is closed for sure, even if not garbage-collected

This PR implements this metamethod

[codecov]

Codecov Report

Merging #78 (e0a8f04) into lua5.4 (ca49b8e) will increase coverage by 1.48%.
The diff coverage is 96.33%.

❗ Current head e0a8f04 differs from pull request most recent head 13a5502. Consider uploading reports for the commit 13a5502 to get more accurate results

@@            Coverage Diff             @@
##           lua5.4      #78      +/-   ##
==========================================
+ Coverage   86.35%   87.83%   +1.48%     
==========================================
  Files          90       93       +3     
  Lines        9920    10240     +320     
==========================================
+ Hits         8566     8994     +428     
+ Misses       1063      956     -107     
+ Partials      291      290       -1     

Impacted Files	Coverage Δ
lib/stringlib/pattern/builder.go	90.79% <ø> (ø)
runtime/debughooks.go	74.28% <40.00%> (+40.95%)	⬆️
lib/iolib/iolib.go	90.97% <64.70%> (-1.22%)	⬇️
runtime/luacont.go	91.00% <93.33%> (+0.95%)	⬆️
runtime/comp.go	93.10% <93.61%> (+0.37%)	⬆️
lib/base/base.go	88.76% <100.00%> (+0.12%)	⬆️
lib/base/warn.go	100.00% <100.00%> (ø)
lib/mathlib/mathlib.go	88.37% <100.00%> (+4.04%)	⬆️
lib/stringlib/format.go	95.04% <100.00%> (+1.03%)	⬆️
lib/stringlib/matching.go	100.00% <100.00%> (ø)
... and 24 more

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