keyboard: support ordinal modifiers

[Pokey Rule]

Some links:

• Search the code base for nextPrev. You'll need to make a new type of token called ordinal that functions identically to nextPrev, but creates an ordinalScope modifier instead of the relativeScope created by nextPrev
• Here is the shape of the ordinal modifier you'll end up creating from our keyboard grammar:

  cursorless/packages/common/src/types/command/PartialTargetDescriptor.types.ts

  Lines 263 to 280 in 5f94e08

   /** 
    * Refer to scopes by absolute index relative to iteration scope, eg "first 
    * funk" to refer to the first function in a class. 
    */ 
   export interface OrdinalScopeModifier { 
     type: "ordinalScope"; 
    
     scopeType: ScopeType; 
    
     /** The start of the range.  Start from end of iteration scope if `start` is negative */ 
     start: number; 
    
     /** The number of scopes to include.  Will always be positive.  If greater than 1, will include scopes after {@link start} */ 
     length: number; 
    
     /** If true, yields individual targets instead of contiguous range. Defaults to `false` */ 
     isEvery?: boolean; 
   }