This is an attempt to address issue #10339.

Background:

Currently, the `editor.tabSize` option does two things - it specifies the width of the tab character and it specifies how many columns to advance when the tab key is pressed. However, there is code in the wild that has a mix of spaces and tabs that expects these two values to be different.

These generally use and indent size of 2 or 4 and spaces are used for indentation until the indent becomes >= 8. The tab character size is excpected to be 8 and groups of 8 spaces are replaced with a tab character. Indent levels end up looking like 2 spaces, 4 spaces, 6 spaces, 1 tab, 1 tab + 2 spaces, and so on.

Implementation:

In the editor options, a new option, `editor.indentSize` is added. This, in conjunction with `editor.tabSize` has the same semantics as `indent_size` and `tab_width` in the well known [EditorConfig specification][1].

> indent_size: a whole number defining the number of columns used for each indentation level and the width of soft tabs (when supported). When set to "tab", the value of tab_width (if specified) will be used.
>
> tab_width: a whole number defining the number of columns used to represent a tab character. This defaults to the value of indent_size and doesn't usually need to be specified.

[1]: editorconfig.org

The new `indentSize` option takes a numeric value or "tab" just as EditorConfig's `indent_size`. The default value is set to "tab" so that current default behavior of VS Code does not change and existing user settings will not break.

When getting the new `indentSize` option programatically, it always returns a numeric value (just as `tabSize` does when set to the deprecated "auto" value).

In the text editor model, a new property is added for `indentSize`. Unlike the configuration options where the value of one property influences the other, In this code `tabSize` now should only mean "the width of the tab character" and `indentSize` should only mean "how may columns is one indent".

The cursor operations and shift command are updated to reflect these new semantics.