Ctrl-d keymap clashes with lsp popup scrolling in insert mode and other insert keymap suggestions

[drybalka]

Ctrl-d in insert mode behaves differently depending on whether an lsp popup is shown or not, which is confusing. The reason, I think, is the keymap clash with popup scrolling. As Ctrl-d is an established scrolling keymap in multiple modes I propose to remap "delete_char_forward" to Ctrl-l. This will make it consistent with the hjkl movements and existing Ctrl-h behavior.

As additional suggestions for improving consistency in insert mappings:

• In pair to to Ctrl-k for "kill_to_line_end" one may use Ctrl-j (instead of Ctrl-u) for "kill_to_line_start". This way all Ctrl-hjkl mappings in the insert mode will have the same delete-in-some-direction meaning. Besides, even though Ctrl-u is a popular binding, it is not beginner friendly (it is not mnemonic and does not have a similar meaning in other modes, e.g., has nothing to do with undo).
• Ctrl-w is an excellent beginner-friendly mnemonic keymapping for deleting a previous word. I think it would improve consistency if deleting a next word were mapped to Ctrl-W or Alt-w instead of Alt-d (which is not really an Alt-ernative for Ctrl-d and is not so easy to remember).
• Ctrl-x is not mnemonic for completions, has nothing to do with x behavior in normal mode, and is even a bit awkward to type. As space seems to be an lsp-related key I think Ctrl-space would be a much better and beginner-friendly default for "completion" (and I think it is even common for vscode).