Refactor Indent Detection to set indentSize to auto rather than tabSize

[ParadoxV5]

I can tell that VSCode historically used tabSize and no indentSize. indentSize is a relatively new concept ((still) incompletely) introduced in #155450, but Indent Detection (and possibly other features as well) is still in the past and not migrated to use indentSize (such as #193811). Problems arise with this inconsistency, such as

• Per-file indentation such as “Detect Indentation” (still) uses tab size rather than indent size  #199630
  (With a whopping (0) upvotes, this bug report will soon get archived as a not-planned feature. At least all but the 2nd bullet point would surface when Add support for mixed tab/spaces indentation #5394 gets attention.)
• Allow setting indent size to an explicit number when it is computed from tab size #200368
  solving Can't set tabSize and indentSize separately #199791 by working the problem around with a new attribute originalIndentSize (and merged with no discussion).
• and possibly more

Request: Finish this mess up and migrate Indent Detection from tabSize: 'auto' to indentSize: 'auto'.

Background: I indent with spaces, yes, but I like my tabs extra long for, well, tabulation, that’s what ‘tab’ stands for.

(Really, if indentSize is still half-baked, why is it in production (stable versions) already?)

vscode/src/vs/editor/common/model/textModel.ts

Line 678 in 8b37750

indentSize: guessedIndentation.tabSize, // TODO@Alex: guess indentSize independent of tabSize