improve reserved word completions #1047

Job	Run time
static analysis (Elixir 1.15.x \| Erlang/OTP 25.x)	23s
mix test windows (Elixir 1.13.x \| Erlang/OTP 22.x)	9m 57s
mix test (Elixir 1.13.x \| Erlang/OTP 22.x)	6m 20s
mix test windows (Elixir 1.13.x \| Erlang/OTP 23.x)	8m 20s
mix test (Elixir 1.13.x \| Erlang/OTP 23.x)	5m 58s
mix test windows (Elixir 1.13.x \| Erlang/OTP 24.x)	7m 11s
mix test (Elixir 1.13.x \| Erlang/OTP 24.x)	4m 29s
mix test windows (Elixir 1.13.x \| Erlang/OTP 25.x)	5m 40s
mix test (Elixir 1.13.x \| Erlang/OTP 25.x)	4m 38s
mix test windows (Elixir 1.14.x \| Erlang/OTP 23.x)	8m 18s
mix test (Elixir 1.14.x \| Erlang/OTP 23.x)	7m 20s
mix test windows (Elixir 1.14.x \| Erlang/OTP 24.x)	6m 31s
mix test (Elixir 1.14.x \| Erlang/OTP 24.x)	4m 27s
mix test windows (Elixir 1.14.x \| Erlang/OTP 25.x)	6m 45s
mix test (Elixir 1.14.x \| Erlang/OTP 25.x)	4m 5s
mix test windows (Elixir 1.15.x \| Erlang/OTP 24.x)	7m 41s
mix test (Elixir 1.15.x \| Erlang/OTP 24.x)	4m 36s
mix test windows (Elixir 1.15.x \| Erlang/OTP 25.x)	5m 37s
mix test (Elixir 1.15.x \| Erlang/OTP 25.x)	4m 10s
	1h 52m 26s

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