api: api_info()['version']

API level is disconnected from NVIM version. The API metadata holds the
current API level, and the lowest backwards-compatible level supported
by this instance.

Release 0.1.6 is the first release that reports the Nvim version and API
level.

    metadata['version'] = {
      major: 0,
      minor: 1,
      patch: 6,
      api_level: 1,
      api_compatible: 0,
      api_prerelease: false,
    }

The API level may remain unchanged across Nvim releases if the API has
not changed.

When changing the API,
    - set NVIM_API_PRERELEASE to true
    - increment NVIM_API_LEVEL (at most once per Nvim version)
    - adjust NVIM_API_LEVEL_COMPAT if backwards-compatibility was broken

api_level_0.mpack was generated from Nvim 0.1.5 with:
    nvim --api-info

1 files changed, 1 insertions, 1 deletions

diff --git a/test/functional/eval/api_functions_spec.lua b/test/functional/eval/api_functions_spec.lua
index 5345c9782d..21dd228145 100644
--- a/test/functional/eval/api_functions_spec.lua
+++ b/test/functional/eval/api_functions_spec.lua
@@ -106,7 +106,7 @@ describe('api functions', function()
 
   it('have metadata accessible with api_info()', function()
     local api_keys = eval("sort(keys(api_info()))")
-    eq({'api_level', 'error_types', 'functions', 'types'}, api_keys)
+    eq({'error_types', 'functions', 'types', 'version'}, api_keys)
   end)
 
   it('are highlighted by vim.vim syntax file', function()