Added streamer.lua

Steamers are akin to that of Telescope's Pickers. A "steamer" is the
class structure used by builtins to create new windows/buffers for the
user to interact with. This code was simply migrated across from
presser@new and refactored to adopt a class-like OOP paradigm.

steamers@new is a wrapper function accessed by builtins to invoke the
class constructor Steamers@new. Allows to greater control and
flexibility of window/buffer options.

presser/steamers.lua

@@ -0,0 +1,78 @@
+local popup = require'plenary.popup'
+local gcm = require'presser.context_manager'
+
+local a = vim.api
+
+local steamers = {}
+
+local Steamer = {}
+Steamer.__index = Steamer
+
+-- @Description: Construct a new instance of a window and buffer using the 'plenary.popup' module.
+-- @Params:
+--    + `_type` ~ string denoting which context manager is owner of the window.
+--    + `placeholder` ~ string allowing for placeholder text to be placed within the buffer when constructed.
+--    + `opts` ~ a table of additional options to be provided when constructing the window.
+-- @Returns: nil. Constructs a new window and stores the window's ID in a global table.
+--
+-- @Dev: This function is to be as generic for creating a window whilst allowing for as much customisation over the
+-- window/buffer which are to be constructed. Users are to access this function so is not part of the module export.
+-- However, it's used by all internal built-ins which are meant to be used by users. Consider a class constructor in
+-- C++, this function is akin to that behaviour and is responsible for constructing the window/buffer and ensuring it
+-- can be tracked by Vim itself.
+function Steamer:new ( _ctx, label, opts )
+  local opts = opts or {}
+  if not opts.placeholder then
+    opts["placeholder"] = ""
+  end
+  
+  local buf_opts = {
+    minwidth = 80, -- getwin_w() / 2 - ((getwin_w() % 2) / 2),
+    borderchars = { "─", "│", "─", "│", "╭", "╮", "╯", "╰" },
+  }
+
+  if opts.window then
+    for k,v in pairs( opts.window ) do
+      buf_opts[k] = v
+    end
+  end
+
+  print("building window: ", opts.placeholder, buf_opts )
+  local obj = popup.create( opts.placeholder, buf_opts )
+  gcm.update( _ctx, { id = obj, what = label, allowed = opts.allowed } )
+  -- :TODO: call update function in context manager
+--  table.insert( manager[_type], obj )
+--  g.presser_buf_ctx = manager
+
+  -- :@Dev: handle for keybindings (TODO: any future stuff below here once window is made)
+  local buf_id = function ()
+    return a.nvim_win_get_buf(obj)
+  end
+
+  -- handle for key bindings
+  if opts.keybinds then
+    for mode, mode_map in pairs( opts.keybinds ) do
+      mode = string.lower(mode)
+
+      for key_bind, key_action in pairs(mode_map) do
+        local key_bind = a.nvim_replace_termcodes(key_bind, true, false, true),
+        a.nvim_buf_set_keymap(buf_id(), mode, key_bind, key_action,
+          { noremap = true, silent = true }
+        )
+      end
+    end
+  end
+end
+
+steamers.new = function ( ctx, opts )
+  -- error if the provided option is not type string
+  assert( type(opts.label) == "string", "Should be string, found type: " .. type(opts.label) ) 
+
+  -- @Dev: function can provide greater wrapper options in future.
+
+  return Steamer:new( ctx, opts.label, opts.steamer )
+end
+
+steamers._Steamer = Steamer
+
+return steamers