repl: add .clearhistory command

[bitpshr]

This pull request adds a .clearhistory REPL command that clears the current session history, both in memory and in the persistent history file when one is configured.

It clears the in-memory history array, resets the navigation index, and truncates the backing file through the open handle. Any pending debounced flush is cancelled first so it cannot rewrite the file after it has been cleared.

Fixes: #63905

[bnb]

This feels a bit off to me. The functionality makes sense, I could see wanting this. But I'm not sure if it's clear when I'd want this vs .clear and the naming is too similar. Is there a more concise name, potentially?

[bitpshr]

@bnb good call. A few options:

• .history clear: reads like history -c in a shell, keeps .clear unambiguous, and leaves room for .history to do more later (listing the entries, for instance). Bare .history would just print a short usage line for now.
• .forget: short and distinct, though on its own it's a little vague about what's being forgotten.
• .resetHistory (or .wipeHistory): still a single command, just renamed off the clear prefix so it stops shadowing .clear, but not really any more concise.

I lean toward .history clear. It's the most concise as a base command and it sidesteps the overlap with .clear entirely.

[SudhansuBandha]

@bnb This functionality will help users to remove commands/contents from previous sessions if they are trying to use same variables or functions with different purpose in subsequent new sessions. Without this functionality user will have to delete/clear contents of the history file to achieve the same.

I would also agree that the name can be changed since it is quite similar to that of clear command.