Insertmode performance improvement

[teslae1]

What this PR does / why we need it:
This is a pr to improve "typing while in insert mode" performance.
This is effectively a refactor since that the changes do not introduce any behavioural changes - only increase performance with existing behaviour.

The changes (per the commit messages) do the following:

• skip sorting and shallow copy when there is only a single cursor
  • it is very often the case that there is only a single cursor
• TypeInInsertMode action gets identified earlier (at iteration 6 instead of 11) at iteration that happens at base.ts line 267
  • this is a notable performance improvement - since several heap allocations and function calls are skipped foreach keystroke when in insert mode and "just typing".
• o(1) lookup instead of o(n)
  • ensures o(1) performance for "isLiteralMode" lookup that is executed at every keystroke

Which issue(s) this PR fixes
No issues has been created that I know of - I just profiled the "typing while in insertmode" code-path and added low-impact optimizations.

Special notes for your reviewer:
Submitting this to contribute to this great extension and fix issues I experience myself.

[J-Fields]

Any reason for moving the methods? It makes it more difficult to tell what changed

[teslae1]

Yes, the only change that was made here was the move itself - only change of location and not of contents - This change causes the "TypeInInsertMode" action to get identified earlier (at iteration 6 instead of 11) at iteration that happens at base.ts line 267

[J-Fields]

Why not use a Set rather than a Map?

[teslae1]

You are right that is a better approach - I changed it

[J-Fields]

Have you measured a performance difference with these changes? I want to be sure we're not prematurely optimizing

[teslae1]

I have tried - but could not create reproduceable measurements. I understand