Create a new template function: capitalize (#5409)

## Description

This creates a new template function that capitalizes the text (make the
first character have upper case and the rest lower case). Example:
```console
beet list -f '%capitalize{$title}'
```

This does not do the same as the *%title* function.

(...)

## To Do

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beets/library.py

@@ -1783,6 +1783,11 @@ def tmpl_upper(s):
         """Convert a string to upper case."""
         return s.upper()
 
+    @staticmethod
+    def tmpl_capitalize(s):
+        """Converts to a capitalized string."""
+        return s.capitalize()
+
     @staticmethod
     def tmpl_title(s):
         """Convert a string to title case."""

docs/changelog.rst

@@ -8,6 +8,8 @@ Changelog goes here! Please add your entry to the bottom of one of the lists bel
 
 New features:
 
+* New template function added: ``%capitalize``. Converts the first letter of
+  the text to uppercase and the rest to lowercase.
 * Ability to query albums with track db fields and vice-versa, for example
   `beet list -a title:something` or `beet list artpath:cover`. Consequently
   album queries involving `path` field have been sped up, like `beet list -a

docs/reference/pathformat.rst

@@ -60,6 +60,7 @@ These functions are built in to beets:
 
 * ``%lower{text}``: Convert ``text`` to lowercase.
 * ``%upper{text}``: Convert ``text`` to UPPERCASE.
+* ``%capitalize{text}``: Make the first letter of ``text`` UPPERCASE and the rest lowercase.
 * ``%title{text}``: Convert ``text`` to Title Case.
 * ``%left{text,n}``: Return the first ``n`` characters of ``text``.
 * ``%right{text,n}``: Return the last ``n`` characters of  ``text``.

test/test_library.py

@@ -629,6 +629,10 @@ def test_upper_case_variable(self):
         self._setf("%upper{$title}")
         self._assert_dest(b"/base/THE TITLE")
 
+    def test_capitalize_variable(self):
+        self._setf("%capitalize{$title}")
+        self._assert_dest(b"/base/The title")
+
     def test_title_case_variable(self):
         self._setf("%title{$title}")
         self._assert_dest(b"/base/The Title")