Follow up: Pull filter bar into its own component

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Summary

The following discussion from !199329 (merged) should be addressed:

• @ms.mondrian started a discussion: (+4 comments)

  suggestion (non-blocking): i think we're slowly reaching the point tree_list is a bit too long to parse. wdyt about pulling this into its own component? could be a very nice follow up?

Implementation Plan

in app/assets/javascripts/repository/file_tree_browser/components/tree_list.vue, gather the markup/computed/const and other related logic for file tree browser filter bar and extract it to a new file. move related tests to a new test file. functionalities of the filter bar should stay the same, including:

• filter file by name
• using keyboard shortcut to focus on filter
• tooltip shows on hover/focus

Context

This is part of a bigger effort to implement a file tree browser (FTB) in the repository. All changes are behind a feature flag repository_file_tree_browser