Prevent Ctrl-C from flushing the command that starts the prompt.

Followup to #20883
Related: #18144

This patch changes the behavior of the default `vim.ui.input` when the user
aborts with `<C-c>`. Currently, it produces an error message + stack and causes
`on_confirm` to not be called. With this patch, `<C-c>` will cause `on_confirm`
to be called with `nil`, the same behavior as when the user aborts with `<Esc>`.
I can think of three good reasons why the behavior should be this way:

1. Easier for the user to understand** It's not intuitive for there to be two
   ways to abort an input dialog that have _different_ outcomes. As a user,
   I would expect any action that cancels the input to leave me in the same
   state. As a plugin author, I see no value in having two possible outcomes for
   aborting the input. I have to handle both cases, but I can't think of
   a situation where I would want to treat one differently than the other.

2. Provides an API that can be overridden by other implementations** The current
   contract of "throw an error upon `<C-c>`" cannot be replicated by async
   implementations of `vim.ui.input`. If the callsite wants to handle the case
   of the user hitting `<C-c>` they need to use `pcall(vim.ui.input, ...)`,
   however an async implementation will instantly return and so there will be no
   way for it to produce the same error-throwing behavior when the user inputs
   `<C-c>`. This makes it impossible to be fully API-compatible with the
   built-in `vim.ui.input`.

3. Provides a useful guarantee to the callsite** As a plugin author, I want the
   guarantee that `on_confirm` will _always_ be called (only catastrophic errors
   should prevent this). If I am in the middle of some async thread of logic,
   I need some way to resume that logic after handing off control to
   `vim.ui.input`. The only way to handle the `<C-c>` case is with `pcall`,
   which as already mentioned, breaks down if you're using an alternative
   implementation.

fix(vim.ui.input): return empty string when inputs nothing

The previous behavior of `vim.ui.input()` when typing <CR> with
no text input (with an intention of having the empty string as input)
was to execute `on_confirm(nil)`, conflicting with its documentation.

Inputting an empty string should now correctly execute `on_confirm('')`.
This should be clearly distinguished from cancelling or aborting the
input UI, in which case `on_confirm(nil)` is executed as before.

Fix #18143

* vim.ui.input is an overridable function that prompts for user input
* take an opts table and the `on_confirm` callback, see `:help vim.ui.input` for more details
* defaults to a wrapper around vim.fn.input(opts)
* switches the built-in client's rename handler to use vim.ui.input by default

Follow up to https://github.com/neovim/neovim/pull/15771

Continuation of https://github.com/neovim/neovim/pull/15202

A plugin like telescope could override it with a fancy implementation
and then users would get the telescope-ui within each plugin that
utilizes the vim.ui.select function.

There are some plugins which override the `textDocument/codeAction`
handler solely to provide a different UI. With custom client commands and
soon codeAction resolve support, it becomes more difficult to implement
the handler right - so having a dedicated way to override the picking
function will be useful.