feat: add case preservation support via custom :s flag P (#3978)

[ronaldocheruiyot]

What this PR does / why we need it:
This PR adds support for preserving case during a substitution command (:s), matching the native behavior found in VSCode's built-in Find and Replace functionality.

Because standard Vim uses the lowercase p flag for printing the last matched line, this introduces a custom uppercase P flag to handle case preservation (e.g., running :%s/foo/bar/P will dynamically transform FOO foo Foo into BAR bar Bar).

It intercepts the substitution loop via a custom utility function that checks the original matched string's text layout (UPPERCASE, lowercase, or Title Case) and applies the identical format to the incoming replacement string before writing it to the document transformer.

Which issue(s) this PR fixes
Fixes #3978

Special notes for your reviewer:

• Added an automated framework unit test under test/cmd_line/substitute.test.ts to validate UPPER, lower, and Title Case execution scenarios simultaneously.
• Compilation is successful locally via yarn build-dev.