Add atomic tabs option

[Timmmm]

This treats tabs faked using spaces as if they were real tabs meaning that you can't select the middle of them. I have added a test, with 100% code coverage over the core logic. I've also tested this fairly extensively with hairy mixed-indentation files and with multiple cursors.

There are a few caveats that I need advice with:

1. I have not used indentSize anywhere (see this comment).

2. I am not 100% sure I added the new atomicTabs setting everywhere it should be. I basically grepped for insertSpaces and added it in the same places (except a few bits where it didn't make sense). But that is loaaads of places so I might have missed some.

3. There is one bg with the mouse handling that I need advice on (not sure I've intercepted the mouse stuff in the right place). Basically if you just click it's fine, but if you select some text and then click in the selection it won't use the atomically snapped position.

This PR fixes #2798

[ghost]

All CLA requirements met.

[Timmmm]

Here is a video showing the behaviour, including the outstanding bug.

[rmunn]

I'd recommend the name "atomic soft tabs" rather than "atomic tabs", as the latter would make some people (including me) think this changed the behavior of tab characters. "Soft tabs" is the usual name, AFAICT, for "hit the Tab key and N space characters get added to the file" feature (where N is, of course, the value of indentSize, because that's what people using a "soft tabs" feature expect to get when they hit the Tab key).

[Timmmm]

The Mac build failed due to a network failure. Is there any way to run it again other than pushing a dummy commit?

[rmunn]

Closing and reopening the PR also retriggers build steps, without needing a dummy commit.

[Cons-Cat]

Amazing to see this! I was recently surprised to find that it isn't already a core feature, but now I really hope it will be.

[Timmmm]

Ok this works perfectly as far as I can tell now. Who do I need to bribe to get it reviewed? :-D

[Cons-Cat]

It's very unfortunate that this isn't receiving the attention it deserves.

[Timmmm]

@alexdima Any chance of a review?

[alexdima]

This looks very good! Thank you! ❤️

I just did some changes on top:

• it's unlucky that you looked for insertSpaces, which is a special kind of editor option, one that lives with the text buffer. I've converted the option to a regular editor option, reducing the changed file count.
• I pushed a slight optimization to avoid walking the leading whitespace again in case of going left.
• small stylistic changes

I'm sorry I didn't get to this sooner.

[Timmmm]

Amazing, thank you so much!

[alexdima]

I've renamed the setting to editor.stickyTabStops since we already had a setting called editor.useTabStops to control the behaviour of backspace / delete for the spaces in the indentation and the feedback from the team was that "atomic" and "soft tabs" were new terms that they didn't hear before.

[Timmmm]

I copied it from Atom

I thought it was a decent enough name - atomicTabStops would be fine too. Atomic in the sense that the tabs can no longer be split.

I'm not sure I understand sticky. Isn't sticky for things that stick? Like a sticky menu that sticks to the top of your screen. Are you thinking that the cursor sticks to the tab stop?

Anyway you guys are the bosses - not really too bothered what it is called as long as it exists! :-D

By the way I made a little video to demo it, in case you want something for the release notes.

[Timmmm]

Demo video:

atomic_tabs.mp4.zip