Make char_begins_new_word more explict & rigorous

This commit aims to fix two things:

- The semi-crypticness of the previous version of char_begins_new_word
- The inconsistency (vs. the rest of the approach) in how the method
  was comparing "top" to "bottom" for interline comparisons, instead of
  "top" to "top", as rightly and helpfully pointed out by @bellma-lilly
  in #840

Based on the unit tests, this shouldn't change the output of
`pdfplumber` in the vast majority of use cases. It might affect some
output in edge-cases, for which I apologize for any inconvenience and
which I hope is balanced out by this more consistent approach's benefits
in the long run.

pdfplumber/utils/text.py

@@ -265,18 +265,76 @@ def merge_chars(self, ordered_chars: T_obj_list) -> T_obj:
     def char_begins_new_word(
         self,
         prev_char: T_obj,
-        next_char: T_obj,
+        curr_char: T_obj,
     ) -> bool:
+        """This method takes several factors into account to determine if
+        `curr_char` represents the beginning of a new word:
+
+        - Whether the text is "upright" (i.e., non-rotated)
+        - Whether the user has specified that horizontal text runs
+          left-to-right (default) or right-to-left, as represented by
+          self.horizontal_ltr
+        - Whether the user has specified that vertical text the text runs
+          top-to-bottom (default) or bottom-to-top, as represented by
+          self.vertical_ttb
+        - The x0, top, x1, and bottom attributes of prev_char and
+          curr_char
+        - The self.x_tolerance and self.y_tolerance settings. Note: In
+          this case, x/y refer to those directions for non-rotated text.
+          For vertical text, they are flipped. A more accurate terminology
+          might be "*intra*line character distance tolerance" and
+          "*inter*line character distance tolerance"
+
+        An important note: The *intra*line distance is measured from the
+        *end* of the previous character to the *beginning* of the current
+        character, while the *inter*line distance is measured from the
+        *top* of the previous character to the *top* of the next
+        character. The reasons for this are partly repository-historical,
+        and partly logical, as successive text lines' bounding boxes often
+        overlap slightly (and we don't want that overlap to be interpreted
+        as the two lines being the same line).
+
+        The upright-ness of the character determines the attributes to
+        compare, while horizontal_ltr/vertical_ttb determine the direction
+        of the comparison.
+        """
+
+        # Note: Due to the grouping step earlier in the process,
+        # curr_char["upright"] will always equal prev_char["upright"].
+        if curr_char["upright"]:
+            inter_tol = self.y_tolerance
+            intra_tol = self.x_tolerance
+
+            inter_attr = "top"
+            intra_attr_min = "x0"
+            intra_attr_max = "x1"
+
+            if self.horizontal_ltr:
+                char_min = prev_char
+                char_max = curr_char
+            else:
+                char_min = curr_char
+                char_max = prev_char
+        else:
+            inter_tol = self.x_tolerance
+            intra_tol = self.y_tolerance
 
-        upright = prev_char["upright"]
-        intraline_tol = self.x_tolerance if upright else self.y_tolerance
-        interline_tol = self.y_tolerance if upright else self.x_tolerance
+            inter_attr = "x0"
+            intra_attr_min = "top"
+            intra_attr_max = "bottom"
+
+            if self.vertical_ttb:
+                char_min = curr_char
+                char_max = prev_char
+            else:
+                char_min = prev_char
+                char_max = curr_char
 
         return bool(
-            (next_char["x0"] > prev_char["x1"] + intraline_tol)
-            or (next_char["x1"] < prev_char["x0"] - intraline_tol)
-            or (next_char["top"] > prev_char["bottom"] + interline_tol)
-            or (next_char["bottom"] < prev_char["top"] - interline_tol)
+            # Intraline test
+            (char_max[intra_attr_min] > char_min[intra_attr_max] + intra_tol)
+            # Interline test
+            or (char_max[inter_attr] > char_min[inter_attr] + inter_tol)
         )
 
     def iter_chars_to_words(