completion changes for execution options

[cscheid]

Execution options are current handled inconsistently. They can be declared either inside or outside an execute key, and they can appear in different contexts.

Outside execute:

context	honored	validated	completed
top-level in project configuration	yes	yes	no
inside specific formats in project configuration	yes	yes	no
top-level in document configuration	yes	yes	no
inside specific formats in document configuration	yes	yes	no
inside code cells with the comment-pipe syntax (#| echo: false, etc)	yes	yes	yes

Inside execute:

context	honored	validated	completed
top-level in project configuration	yes	yes	yes
inside specific formats in project configuration	yes	no	no
top-level in document configuration	yes	yes	yes
inside specific formats in document configuration	yes	no	no
inside code cells with the comment-pipe syntax (#| echo: false, etc)	NA	NA	NA

[cscheid]

(cc @cwickham)