feat: has('terminfo')

Problem:
No way to detect at runtime if the build includes unibilium (or whatever
terminfo layer we swap it with later).

Solution:
Support `has('terminfo')`.

test/functional/vimscript/has_spec.lua

@@ -64,6 +64,17 @@ describe('has()', function()
     end
   end)
 
+  it('"terminfo"', function()
+    -- Looks like "HAVE_UNIBILIUM ", "HAVE_UNIBILIUM=1", "HAVE_UNIBILIUM off", ….
+    -- Capture group returns the "1"/"off"/….
+    local build_flag =
+      vim.trim((n.exec_capture('verbose version'):match('HAVE_UNIBILIUM([^-]+)') or ''):lower())
+    -- XXX: the match() above fails in CI so currently we assume CI always builds with unibilium.
+    local is_enabled = t.is_ci()
+      or not (build_flag == '' or build_flag == 'false' or build_flag == '0' or build_flag == 'off')
+    eq(is_enabled and 1 or 0, fn.has('terminfo'))
+  end)
+
   it('"wsl"', function()
     local is_wsl = vim.uv.os_uname()['release']:lower():match('microsoft') and true or false
     if is_wsl then