Adds nvim_get_hl_by_name/by_id

...in order to retrieve highlights.

Added test/functional/api/highlight_spec.lua
HL_NORMAL is not really a good name, since it's more like an empty attribute than the normal's one.
If one pays attention, syn_cterm_attr2entry is never called with attr=0 because it's always special cased before.
I suggest in subsequent PRs we remove the ATTR_OFF and just insert an EMPTY ATTR/RESET_ATTR/UNINITIALIZED for id 0.

test/functional/api/highlight_spec.lua

@@ -0,0 +1,54 @@
+
+local helpers = require('test.functional.helpers')(after_each)
+local clear, nvim = helpers.clear, helpers.nvim
+local Screen = require('test.functional.ui.screen')
+local eq, eval = helpers.eq, helpers.eval
+local command = helpers.command
+local ok = helpers.ok
+local meths = helpers.meths
+
+
+describe('highlight api', function()
+
+  before_each(function()
+    clear('--cmd', 'set termguicolors')
+  end)
+
+  it("nvim_get_hl_by_id", function()
+    local expected_hl = { background = Screen.colors.Yellow,
+                          foreground = Screen.colors.Red,
+                          special = Screen.colors.Blue
+                        }
+
+    command('hi NewHighlight guifg=red guibg=yellow guisp=blue')
+
+    local hl_id = eval("hlID('NewHighlight')")
+    eq(expected_hl, nvim("get_hl_by_id", hl_id))
+
+    -- assume there is no hl with 30000
+    local err, emsg = pcall(meths.get_hl_by_id, 30000)
+    eq(false, err)
+    ok(string.find(emsg, 'Invalid highlight id') ~= nil)
+  end)
+
+  it("nvim_get_hl_by_name", function()
+    local expected_hl = { background = Screen.colors.Yellow,
+                          foreground = Screen.colors.Red }
+
+    -- test "Normal" hl defaults
+    eq({}, nvim("get_hl_by_name", 'Normal'))
+
+    command('hi NewHighlight guifg=red guibg=yellow')
+    eq(expected_hl, nvim("get_hl_by_name", 'NewHighlight'))
+
+    command('hi Normal guifg=red guibg=yellow')
+    eq(expected_hl, nvim("get_hl_by_name", 'Normal'))
+    local err, emsg = pcall(meths.get_hl_by_name , 'unknown_highlight')
+    eq(false, err)
+    ok(string.find(emsg, 'Invalid highlight name') ~= nil)
+  end)
+
+
+
+end)
+