Add keyword for alternatives in a match construct

edit @pokey:

As discussed in https://github.com/cursorless-dev/cursorless/issues/579#issuecomment-1161627378, we’d like to proceed as follows:

• #1172
• #1173
• #1174
• #1175
• #1176

Original

There currently is no keyword for selecting alternatives in a match construct. Examples of match constructs are:

• Python

  match x:
  case Alt1: exp1
  …
  case AltN: expN
  

• Haskell

  f = case x of
    Alt1 -> exp1
    …
    AltN -> expN
  

  or

  f Alt1 = exp1
  …
  f AltN = expN
  

We probably want three keywords:

• matched to select the value matched on, i.e., x;
• <ordinal> pattern to select the i’th pattern, i.e., Alt1 to AltN above; and
• <ordinal> case to select the i’th case, i.e., exp1 to expN above.

Unmodified pattern and case should probably to the scopes relative to the named mark or cursor, i.e., pattern refers to Alt1 if the mark or cursor is in either Alt1 or exp1, and similarly for case.

[purpleP]

@pokey I like your proposal.

[pokey]

Ok I’ve edited my comment above with some new terms after a discussion with @AndreasArvidsson and @phillco. Tl;dr:

• "case" became "branch"
• "condition" became "subject" or not supported
• "name" added synonyms "key" and "condition"

See the updated comment for discussion