is_diary_file(): use filereadable() to check a single file

Intended to address #1267 - "get_diary_files() takes very long to
execute on Windows - this leads to very slow link-creation using the
Enter key".

In theory, this should be a performance improvement for link creation on
systems with very slow filesystem access (this came up under Windows, but
ought to be a general optimization).  It does nothing for generating the
entire list of diary files, which by nature has to get the whole list
anyway.

  - Uses a filereadable() call against a path instead of checking the
    results of get_diary_files() for most calls.

  - Introduces is_among_diary_files() for the case where you want to
    check against a get_diary_files() list.

  - is_diary_file() will pass through to is_among_diary_files() if it gets
    invoked with a second parameter, to retain backwards compatibility on
    the off chance anyone has scripted against this behavior.  I'm not sure
    this is necessary, but trying to be cautious with the implicit API
    here.

  - Includes a test/api_base_is_diary_file.vader

My original version of this change assumed that input would be in the
form of "2023-01-01", instead of a fully-qualifed filename.  As it is,
it expects a full path to the file, in keeping with the list returned by
vimwiki#diary#get_diary_files().

Bug: #1267

test/api_base_is_diary_file.vader

@@ -0,0 +1,25 @@
+# Test vimwiki#base#is_diary_file() for various inputs.
+
+Execute (Check known good diary file):
+  VimwikiIndex 1
+  let link_infos = vimwiki#base#resolve_link('diary:2020-07-22')
+  Assert vimwiki#base#is_diary_file(link_infos.filename)
+
+Execute (Check known good diary file using is_diary_file, with list of all diary files - legacy interface):
+  VimwikiIndex 1
+  let link_infos = vimwiki#base#resolve_link('diary:2020-07-22')
+  let diary_file_paths = vimwiki#diary#get_diary_files()
+  Assert vimwiki#base#is_diary_file(link_infos.filename, diary_file_paths)
+
+Execute (Check known good diary file use is_among_diary_files, with list of all diary files):
+  VimwikiIndex 1
+  let link_infos = vimwiki#base#resolve_link('diary:2020-07-22')
+  let diary_file_paths = vimwiki#diary#get_diary_files()
+  Assert vimwiki#base#is_among_diary_files(link_infos.filename, diary_file_paths)
+
+Execute (Check for nonexistent diary file):
+  VimwikiIndex 1
+  Assert !vimwiki#base#is_diary_file('not-a-diary-file')
+
+Execute (Clean):
+  call ReloadVimwiki()