Add: org-ql-block

Usable as an Org Agenda custom command block type.  Inspired by
@pestctrl's custom config:

84c557982a/config-org-new.org (my-own-agenda-renderer)

README.org

@@ -59,11 +59,11 @@ More examples are available in [[examples.org]].
 The functionality provided may be grouped by:
 
 +  Interactive commands :: ~org-ql-search~
-+  Non-interactive functions and macros :: ~org-ql~ (macro), ~org-ql-select~ (function), ~org-ql-query~ (function), and ~org-ql-agenda~ (macro)
++  Non-interactive functions and macros :: ~org-ql~ (macro), ~org-ql-select~ (function), ~org-ql-query~ (function), ~org-ql-block~ (agenda function), and ~org-ql-agenda~ (macro)
 
 Alternatively, they may be grouped by:
 
-+  Showing an agenda-like view :: ~org-ql-search~ (command), and ~org-ql-agenda~ (macro)
++  Showing an agenda-like view :: ~org-ql-search~ (command), ~org-ql-block~ (agenda function), and ~org-ql-agenda~ (macro)
 +  Returning a list of matches or acting on them :: ~org-ql~ (macro), ~org-ql-select~ (function), and ~org-ql-query~ (function)
 
 Feedback on these APIs is welcome.  Eventually, after being tested and polished, they will be considered stable.
@@ -141,6 +141,30 @@ Arguments are listed next to predicate names, where applicable.
 
 *** Agenda-like views
 
+**** Function: ~org-ql-block~
+
+For use as a custom agenda block type in ~org-agenda-custom-commands~.  For example, you could define a custom series command like this, which would list all priority A items tagged =Emacs= with to-do keyword =SOMEDAY=, followed by the standard agenda view, in a single buffer:
+
+#+BEGIN_SRC elisp
+  (setq org-agenda-custom-commands
+        '(("ces" "Custom: Agenda and Emacs SOMEDAY [#A] items"
+           ((org-ql-block '(and (todo "SOMEDAY")
+                                (tags "Emacs")
+                                (priority "A")))
+            (agenda)))))
+#+END_SRC
+
+Which would be equivalent to a ~tags-todo~ search like this:
+
+#+BEGIN_SRC elisp
+  (setq org-agenda-custom-commands
+        '(("ces" "Custom: Agenda and Emacs SOMEDAY [#A] items"
+           ((tags-todo "PRIORITY=\"A\"+Emacs/!SOMEDAY")
+            (agenda)))))
+#+END_SRC
+
+However, the ~org-ql-block~ version runs in about 1/5th the time.
+
 **** Macro: ~org-ql-agenda~
 
 This macro is like ~org-ql~, but it presents matching entries in an Agenda-like view.  It's compatible with [[https://github.com/alphapapa/org-super-agenda][org-super-agenda]], which provides grouping.  For example:
@@ -314,6 +338,7 @@ Expands into a call to ~org-ql-select~ with the same arguments.  For convenience
 +  Bare strings like ~"string"~ can be used in queries, which are converted to ~(regexp "string")~ automatically.
 +  Selector ~(regexp)~ accepts multiple regexps to test.
 +  Macro ~org-ql~ and functions ~org-ql-query~ and ~org-ql-select~ now also accept a comparator function in their ~:sort~ argument.
++  Function ~org-ql-block~, which works as an Org Agenda series/composite/block command, usable in custom agenda commands defined in variable ~org-agenda-custom-commands~.  (Inspired by [[https://github.com/pestctrl/emacs-config/blob/84c557982a860e86d6f67976a82ea776a7bd2c7a/config-org-new.org#my-own-agenda-renderer][Benson Chu's config]].)
 
 *Changed*
 +  Function ~org-ql-query~ renamed to ~org-ql-select~.  ~org-ql-query~ now refers to a new function.