wit: Reserve doc-comment "annotation" syntax

In order to allow for future strict parsing of doc comment annotations, we could reserve a pattern for annotations now (without necessarily defining any valid annotations).

One common approach is @annotations, used by JSDoc, Javadoc, and others. An example of how that could look in WIT (once specified):

/// Runs the command.
/// @param args - A list of command line arguments.
/// @returns `ok` if the command completes successfully.
export run: func(args: list<string>) -> result

We could reserve the pattern e.g. "@ at the start but not middle of a word" (regexp /\b@[a-z]/) or something similar.

Start with the WIT doc-comment syntax and the proposed /\b@[a-z]/ pattern. Compare the possible word-boundary behavior and decide how annotation-like text should be recognized without defining valid annotations. Done means the reserved pattern and its scope are specified clearly enough for future strict parsing.