feat: add TOML codec and envc format support

New toml package (pelletier/go-toml/v2) matching yaml/json options and merge flow.
envc convert/get/merge accept --from/--to toml; fixture and CLI tests; docs refresh.

Co-authored-by: Cursor <cursoragent@cursor.com>

fixtures/identity/service.toml

@@ -0,0 +1,12 @@
+[service]
+name = "my-service"
+
+[service.sub-service]
+name = "abc"
+key = "abs"
+timeout = 30
+enabled = true
+
+[service.database]
+url = "postgres://localhost:5432/db"
+pool-size = 10