Add goos.environ() function

[tatzyr]

Closes #73

Changes

• Add goos.environ() function that returns all environment variables as a Lua table
• Add tests and documentation

Usage

local goos = require("goos")
local env = goos.environ()
print(env.PATH)
print(env.HOME)

-- Iterate all
for key, value in pairs(env) do
    print(key .. "=" .. value)
end

[vadv]

Please add tests for corner cases like this:
export ENV_VAR=TEST=1

[tatzyr]

Thanks for the review. Added just 2 corner cases.

Skipped tests for special or non-ASCII env vars since os.Environ() handles them correctly.

Used os.Setenv + defer instead of t.Setenv() due to Go 1.16 in ci.yaml.

[vadv]

https://github.com/vadv/gopher-lua-libs/releases/tag/v0.8.0

thank you @tatzyr !