ci: add shfmt option to CI workflow #121

Job	Run time
ci / Change Detection	6s
ci / StyLua	0s
ci / Release Please	0s
ci / shfmt	0s
ci / Prettier	0s
ci / Setup Matrix	0s
ci / Lua Language Server	-1s
ci / Markdownlint	-1s
ci / Detect Metadata	-1s
ci / Run Tests (${{ matrix.os }}, ${{ matrix.lua-version }})	-1s
ci / Publish to LuaRocks	-1s
ci / Luacheck	-1s
	0s

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