Zoekt code search always performs regex search

What does this MR do and why?

For exact search we don't need to wrap the query term with "", we just need to escape the regex.

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Before	After

How to set up and validate locally

• Configure a repo for zoekt
• Perform a zoekt code search for regex true with some keyword containing the regex metacharacter. It should perform the regex search
• Perform a zoekt code search for regex false with some keyword containing the regex metacharacter. It should not perform the regex search

Related to #474852 (closed)