Add only_files argument for lsp_save_all command

[jwortmann]

Closes #2371 (?)

Using this argument, you can add a key binding (or menu entry or command palette entry) to save all files, without being bothered by save dialogs for new buffers.

Also did a small adjustment to the caption of the example key binding, to make it clear that lsp_save_all only saves views for which a language server is running.

[rchl]

Looks good but I'm not sure about resolving #2371 (as you are also, I guess).

1. It requires separate, manual shortcut press
2. It saves all unsaved files rather than just the ones that were affected by rename.

That said, it might do well 99% of the time.

[jwortmann]

Right, I made this PR because you mentioned this as an alternative solution to the linked issue, and I thought it would be an easy implementation and a nice little improvement for the "Save all" behavior in general.

I don't really think that adding a new setting to automatically save after workspace edits would be justified. But just to throw in an idea for the case of automatically saving; then it should probably first check all the open files which already had unsaved modifications before, and exclude those files from saving.

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A bit unrelated, but while looking through the implementations, I saw that the lsp_save command should maybe accept the arguments of the built in save command (async, encoding, quiet) and just pass them through when calling built-in save. Then async should be specified in the key binding arguments, instead of being hardcoded in the code. Similar to how the built-in key binding works (Windows example):

{ "keys": ["ctrl+s"], "command": "save", "args": { "async": true } },