gopls/completion: prepend space when completing right after "//" before a declaration

[abhay1999]

When a user places the cursor immediately after "//" (no space) on a
line directly above a declaration, gopls already suggests the
declaration name as a completion candidate. However the inserted text
lacked the conventional space, producing "//Name" instead of the
proper Go doc comment form "// Name".

Detect this case in populateCommentCompletions (cursor at slash+2
with comment text exactly "//") and set a new
commentNeedsLeadingSpace flag on the completionContext. item() checks
the flag and prepends a space to the insert text (and snippet), so
accepting the completion yields "// Name".

A marker test is added to comment.txt asserting that a function name
is offered when the cursor is right after "//" directly above its
declaration. All existing completion marker tests continue to pass.

Fixes golang/go#76374

[gopherbot]

This PR (HEAD: 254d6e0) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/tools/+/762540.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.