codegen: bracket-quote Lua-keyword field names

The plugin emitted bare-identifier field names in three positions:
the `M.<Type>_fields` table key, the inline encoder's `v = t.<name>`
load, and the inline decoder's `result.<name> = ...` store. When a
proto field name collided with a Lua reserved word the generated
`*_pb.lua` failed to load with `'(' expected near '<keyword>'`.

The real-world hit is pprof's profile.proto, which declares
`repeated Function function = 5` — that breaks all three emit sites.

Route every user-named identifier through new luaTableKey /
luaFieldAccess helpers that bracket-quote reserved words. Covers
field_names + oneof descriptors, oneof presence pre-pass, inline
encode/decode field bodies (including repeated and map paths), and
optional has/clear accessors.