feat(vim.version): more coercion with strict=false

Problem:
"tmux 3.2a" (output from "tmux -V") is not parsed easily.

Solution:
With `strict=false`, discard everything before the first digit.

- rename Semver => Version
- rename vim.version.version() => vim.version._version()
- rename matches() => has()
- remove `opts` from cmp()

test/functional/terminal/buffer_spec.lua

@@ -202,7 +202,7 @@ describe(':terminal buffer', function()
 
     -- Save the buffer number of the terminal for later testing.
     local tbuf = eval('bufnr("%")')
-    local exitcmd = helpers.is_os('win')
+    local exitcmd = is_os('win')
       and "['cmd', '/c', 'exit']"
       or "['sh', '-c', 'exit']"
     source([[