feat: tostring(vim.version())

Problem:
tostring(vim.version()) returns "table: 0x…".

Solution:
Modify vim.version() to return a string prerelease instead of a boolean.

Fix #23863

runtime/lua/vim/version.lua

@@ -425,7 +425,7 @@ setmetatable(M, {
   __call = function()
     local version = vim.fn.api_info().version
     -- Workaround: vim.fn.api_info().version reports "prerelease" as a boolean.
-    version.prerelease = version.prerelease or nil
+    version.prerelease = version.prerelease and 'dev' or nil
     return setmetatable(version, Version)
   end,
 })

test/functional/lua/version_spec.lua

@@ -17,6 +17,18 @@ describe('version', function()
     eq({ major = 42, minor = 3, patch = 99 }, exec_lua("return vim.version.parse('v42.3.99')"))
   end)
 
+  it('version() returns Nvim version', function()
+    local expected = exec_lua('return vim.fn.api_info().version')
+    local actual = exec_lua('return vim.version()')
+    eq(expected.major, actual.major)
+    eq(expected.minor, actual.minor)
+    eq(expected.patch, actual.patch)
+    eq(expected.prerelease and 'dev' or nil, actual.prerelease)
+
+    -- tostring() #23863
+    matches([[%d+%.%d+%.%d+]], exec_lua('return tostring(vim.version())'))
+  end)
+
   describe('_version()', function()
     local tests = {
       ['v1.2.3'] = { major = 1, minor = 2, patch = 3 },