feat(build): build.zig MVP: build and run functionaltests on linux

NEW BUILD SYSTEM!

This is a MVP implementation which supports building the "nvim" binary,
including cross-compilation for some targets.
As an example, you can build a aarch64-macos binary from
an x86-64-linux-gnu host, or vice versa

Add CI target for build.zig currently for functionaltests on linux
x86_64 only

Follow up items:

-  praxis for version and dependency bumping
-  windows 💀
-  full integration of libintl and gettext (or a desicion not to)
-  update help and API metadata files
-  installation into a $PREFIX
-  more tests and linters

test/functional/lua/vim_spec.lua

@@ -2100,9 +2100,9 @@ describe('lua stdlib', function()
     eq(false, fn.luaeval "vim.v['false']")
     eq(NIL, fn.luaeval 'vim.v.null')
     matches([[attempt to index .* nil value]], pcall_err(exec_lua, 'return vim.v[0].progpath'))
-    eq('Key is read-only: count', pcall_err(exec_lua, [[vim.v.count = 42]]))
-    eq('Dict is locked', pcall_err(exec_lua, [[vim.v.nosuchvar = 42]]))
-    eq('Key is fixed: errmsg', pcall_err(exec_lua, [[vim.v.errmsg = nil]]))
+    matches('Key is read%-only: count$', pcall_err(exec_lua, [[vim.v.count = 42]]))
+    matches('Dict is locked$', pcall_err(exec_lua, [[vim.v.nosuchvar = 42]]))
+    matches('Key is fixed: errmsg$', pcall_err(exec_lua, [[vim.v.errmsg = nil]]))
     exec_lua([[vim.v.errmsg = 'set by Lua']])
     eq('set by Lua', eval('v:errmsg'))
     exec_lua([[vim.v.errmsg = 42]])
@@ -2111,7 +2111,10 @@ describe('lua stdlib', function()
     eq({ 'one', 'two' }, eval('v:oldfiles'))
     exec_lua([[vim.v.oldfiles = {}]])
     eq({}, eval('v:oldfiles'))
-    eq('Setting v:oldfiles to value with wrong type', pcall_err(exec_lua, [[vim.v.oldfiles = 'a']]))
+    matches(
+      'Setting v:oldfiles to value with wrong type$',
+      pcall_err(exec_lua, [[vim.v.oldfiles = 'a']])
+    )
     eq({}, eval('v:oldfiles'))
 
     feed('i foo foo foo<Esc>0/foo<CR>')