Add NB_SEARCH_UTILITY environment variable

[mweichert]

This pull request introduces a new environment variable, _NB_SEARCHUTILITY, into the notebook search functionality. The changes are compact yet impactful, involving just 4 lines of additions with no deletions.

Key Changes:

• Added a check for an existing _search_utility variable.
• If _search_utility is not set, it now defaults to the value of the new _NB_SEARCHUTILITY environment variable.

Purpose: The introduction of _NB_SEARCHUTILITY aims to enhance our notebook's search capabilities by allowing for dynamic utility configuration. This is particularly useful for environments where the search utility might differ or where a user prefers a custom search method.

Benefits:

• A user doesn't always have to recall to specify their favorite search tool using the --tool or --utility option
• Different search tools can be used browser as well when using nb browse. This was my primary motivation for this PR.

Testing: The changes are minimal and straightforward, focusing solely on environment variable management. I've tested the following:

• The notebook search functions as expected when NB_SEARCH_UTILITY is set.
• The default behavior remains unaffected when NB_SEARCH_UTILITY is not set.

Looking forward to feedback and suggestions from the team!

[xwmx]

@mweichert Thanks! I added NB_SEARCH_TOOL as the primary name for the variable for consistency with other variable names. I still have omitted it from the documentation. There are some features that are still only implemented in git grep, such as —and queries.