Only read text files that look like entries when opening folder journal (#1697)

* Add text file that should be ignored to basic test folder journal. Makes tons of tests fail
* Add additional files that should be ignored by FolderJournal
* Ignore all files in folder journal except year/month/day.txt
* Completely remake get_files in FolderJournal:
- move get_files into FolderJournal class and add underscore prefix
- create iterables to get for year/month folders and day files
- make year/month/day file reading strict: only exact expected months and days out of all possible months and days
* Restore accidentally-deleted self.sort() line
* Use match instead of string comparison to be os-agnostic
* Explicitly declare static methods
* Filter with glob first for max performance
* Explicitly check for valid dates in FolderJournal and add unit test
* Remove unneeded jrnl import
* Clean up method comment and add type hints
* Add is_valid_date unit test
* Elucidate comment

Co-authored-by: Jonathan Wren <jonathan@nowandwren.com>

jrnl/time.py

@@ -89,3 +89,11 @@ def parse(
     if dt.days < -28 and not year_present:
         date = date.replace(date.year - 1)
     return date
+
+
+def is_valid_date(year: int, month: int, day: int) -> bool:
+    try:
+        datetime.datetime(year, month, day)
+        return True
+    except ValueError:
+        return False