fix(agenda): make spans work like emacs (#915)

Currently, view month and view day both shift the cursor to and start
the view's span from the start day (Monday by default). In emacs, all
view changes take into account the agenda date that the cursor is on,
and then choose the beginning date accordingly. These changes aim to
mimic emacs behavior by
1. modifying all view changes to stop moving the cursor
2. displaying the W number in all spans types
3. change the first day of the view to be the first day of the span

Reuse existing function OrgDate:start_of(span)

Ensure start is on user configured start week for initial
week view as well

Co-authored-by: Kristijan Husak <husakkristijan@gmail.com>

Author: sahinf

lua/orgmode/agenda/types/agenda.lua

@@ -133,8 +133,18 @@ function OrgAgendaType:change_span(span)
       return
     end
   end
+  local from = nil
+  if self.view and self.view:is_in_range() then
+    local agenda_line = self:get_line(vim.fn.line('.'))
+    local metadata = agenda_line and agenda_line.metadata or {}
+    ---@type OrgDate
+    local cursor_date = metadata.agenda_day or (metadata.agenda_item and metadata.agenda_item.date)
+    if cursor_date and type(span) == 'string' then
+      from = cursor_date:start_of(span)
+    end
+  end
   self.span = span
-  self:_set_date_range()
+  self:_set_date_range(from)
   return self
 end
 
@@ -430,11 +440,13 @@ end
 function OrgAgendaType:_set_date_range(from)
   local span = self.span
   from = from or self.from
-  local is_week = span == 'week' or span == '7'
-  if is_week and self.start_on_weekday then
-    from = from:set_isoweekday(self.start_on_weekday)
-  end
 
+  if self.start_on_weekday then
+    local is_week = span == 'week' or span == '7'
+    if is_week and from:is_same(Date.today(), 'week') then
+      from = from:set_isoweekday(self.start_on_weekday)
+    end
+  end
   local to = nil
   local modifier = { [span] = 1 }
   if type(span) == 'number' then
@@ -463,9 +475,7 @@ function OrgAgendaType:_get_title()
     span = string.format('%d days', span)
   end
   local span_number = ''
-  if span == 'week' then
-    span_number = string.format(' (W%s)', self.from:get_week_number())
-  end
+  span_number = string.format(' (W%s)', self.from:format('%V'))
   return utils.capitalize(span) .. '-agenda' .. span_number .. ':'
 end