Run pcase-defmacro for interfaces

[leungbk]

Closes #2285.

An alternate approach is to write a separate wrapper entirely (untested):

(defmacro lsp-dcase (expr-val &rest clauses)
  "Destructure EXPR-VAL upon reaching the first matching clause,
then execute the corresponding body form."
  (-when-let (((bindings body) . remaining-clauses) clauses)
    `(-if-let (,bindings ,expr-val)
         (progn ,@body)
       (lsp-dcase ,expr-val ,remaining-clauses))))

But using pcase-defmacro instead lets us use the existing pcase patterns, most notably the seq pattern.

[yyoncho]

Looks very good! Consider adding a unit test for that.

[leungbk]

OK, I added some tests.

[yyoncho]

OK, I added some tests.

Thank you. FYI the reason I am requesting tests is that we already have a good test suite for that particular feature and it is relatively straightforward to add them.