fix: the self-insert command in vi-mode should not insert un-printable char while flushing the pending-keys

[sakurawald]

In pr: #1691, we flush the pending-keys for self-insert command in vi-insert-mode, so that we can define keymap like (define-key lem-vi-mode:*insert-keymap* "j k" 'lem-vi-mode/commands:vi-normal).

The effect of this define-key is:

1. Press j k in vi-insert-mode will the lem-vi-mode/commands:vi-normal command.
2. Press j <second-key> in vi-insert-mode will self-insert j and self-insert <second-key>.

However, in the vim editor, if the <second-key> is NOT print-able key, then it will quit the key pending state, and will not self-insert the <second-key>.

A case is: press j and Backspace.
This pr fix this edge-case.

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NOTE:
This pr has some minor difference from the vim. The named-key is not identical to print-able-key, taken Tab for example.
I didn't add the function printable-key-sym-p or un-printable-key-sym-p, because it is hard to decide whether a key is print-able or not.

The difference is minor, and sufficient in daily use.

[sakurawald]

The vi-pre-command-hook calles the pre-command-hook function.
However, i would like to write this feature in on-command-hook and using :around method-combination to do the effect. (The :around gives the possibility to cancel the self-insert j, if the <second-key> is un-printable-key.)

[sakurawald]

Another joint to handle the pending-keys flushing for self-insert command is to use (defmethod execute :around (mode (command self-insert) argument), so that we can cancel the executing of self-insert command, if the <second-key> is un-printable-key.
However, this method only support to specify the major mode.

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I think it's ready to be merged.

[sakurawald]

Currently, for self-insert command in vi-mode, we flush the keys in (cdr this-command-keys) into the buffer. (We do this via the post-command-hook in vi-insert-mode)

Since the emacs-mode seldom use pure alpha letters to define-key. (Emacs define-keys using alpha-letter with modifier keys), and vi-mode does define some keys using pure alpha-letter like define jk in vi-insert-mode).
I impl the flushing function in post-command-hook of vi-mode, not the global executing :around for self-insert command.