[MahbodHastam]

What this PR does / why we need it:

In Vim, <Up>/<Down> on the command line recall only the history entries that start with what you've typed so far (:help c_<Up>), while <C-p>/<C-n> browsw history unfiltered (:help c_CTRL-P).

Changes:

• <Up>/<Down> skip history entries that don't start with the text before the cursor. This applies to both : command history and //? search history.
• <C-p>/<C-n> are unchanged (unfiltered), matching Vim.
• Moving past the newest entry now restores the text you've typed. Previously it was lost because historyIndex was initialized to history.length instead of undefined, so the typed text was never saved. The existing test had a TODO for this , which is now resolved.
• If you edit a recalled entry and press <Up> again, the edited text becomes the new prefix.

Example (from #3951): search /abc, then /def, then type /a and press <Up>. Before: /def. After: /abc

Which issue(s) this PR fixes
Fixes #3951

Special notes for your reviewer:

• The <Up> and <C-p> actions are split into separate classes in src/actions/commands/commandLine.ts so each can pass matchPrefix.
• Tests added in test/cmd_line/command.test.ts; the existing <C-p>/<C-n> test now asserts :w| instead of :|.