leverage PATH for locating local executables (#283)

1. This is a breaking change for the following servers, which will have to be
reinstalled:

 - ltex
 - clangd

2. This is a breaking change for users who reach into the default options (for
   example via server:get_default_options()) to access the `cmd` property.
   nvim-lsp-installer no longer provides the `cmd` (except in a few
   instances), but instead provides an amended PATH which allows neovim's LSP
   client to locate the locally installed executable.

   To access the `cmd`, simply access it via lspconfig instead, for example
   like so:

    local default_config = require("lspconfig.server_configurations.rust_analyzer").default_config
    print("I can now access the cmd governed by lspconfig:", default_config.cmd)

3. This is a breaking change for 3rd party use cases that makes use of the
   `executable()` APIs (e.g., `npm.executable(root_dir, "tsserver")`). The
   recommended usage is to instead to use the canonical name of the command
   ("tsserver"), while providing an amended PATH, for example:

    local npm = require("nvim-lsp-installer.installers.npm")
    local server = server.Server:new {
        ...,
        root_dir = root_dir,
        installer = npm.packages { "tsserver" },
        default_options = {
            cmd = { "tsserver" },
            cmd_env = npm.env(root_dir),
        }
    }

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