feat(#44): support env var specification

[barrettruth]

support env var spec like so:

ft('cpp'):fmt('clang-format'):env({ A = 'b' })

maintains existing env vars (does not override other than what's specified)

[glepnir]

env just receive a param env table<string, string> . if vim.tbl_count(env) == 0 return then we can se tool.env == env add a validate first by using vim.validate for param env.

[barrettruth]

Attempted to resolve in next commit. Do the table keys and values also need to be validates like so?

    for k, v in pairs(env) do
      vim.validate({
        k = { k, 'string' },
        v = { v, 'string' }
      })
    end

[barrettruth]

Also note that we cannot set tool.env = env because uv.os_environ() returns the environment like:

{ key = val }

and uv.spawn() needs the env like:

{ 'key=val' }

[glepnir]

hmm why we need these codes. just tool.env = env ?

[barrettruth]

I say that above:

Also note that we cannot set tool.env = env because uv.os_environ() returns the environment like:

{ key = val }
and uv.spawn() needs the env like:

{ 'key=val' }

[glepnir]

that's make sense but do we need extend os_environ ?

[barrettruth]

Yes. I believe applying an env override removes all environment variables from the uv.spawn() process or something.

As evidence, it just doesn't work with spawning certain processes (like prettierd, for example) and breaks. Keeping uv.os_environ() fixes it. null-ls also does it.