ci: Adjust GitHub Actions workflow "update lexicon" to make PR instead of pushing to main branch

[shuuji3]

follow up for #36

The CI failed because of the branch protection rule where the workflow cannot directly push the main branch. Also, it's better to create a PR so that we can track the upstream lexicon update well.

CI log

Run git config user.name "github-actions[bot]"
[main 9191114] chore: update lexicons.ts
 1 file changed, 18 insertions(+), 2 deletions(-)
remote: error: GH006: Protected branch update failed for refs/heads/main.        
remote: 
remote: - Changes must be made through a pull request.        
remote: 
remote: - 2 of 2 required status checks are expected.        
remote: 
remote: - You're not authorized to push to this branch. Visit https://docs.github.com/repositories/configuring-branches-and-merges-in-your-repository/managing-protected-branches/about-protected-branches for more information.        
To https://github.com/tsky-dev/tsky
 ! [remote rejected] main -> main (protected branch hook declined)
error: failed to push some refs to 'https://github.com/tsky-dev/tsky'
Error: Process completed with exit code 1.

https://github.com/tsky-dev/tsky/actions/runs/12791155164/job/35658613653