Add .curve_edges, use by default in table-detect.

Most of the groundwork was already there to add a PDF/Page.curve_edges
property. And, inspired, by
https://github.com/jsvine/pdfplumber/discussions/858 and related issues,
we now include 0/90/180/270-degree oriented curve segments into the
default table-detection strategy. As before, you can still switch to the
"lines_strict" to use only lines defined as such (rather than also using
rect and curve edges).

tests/test_table.py

@@ -211,3 +211,16 @@ class Test(unittest.TestCase):
             }
             assert page.extract_table(table_settings)
             assert page.extract_tables(table_settings)
+
+    def test_table_curves(self):
+        # See https://github.com/jsvine/pdfplumber/discussions/808
+        path = os.path.join(HERE, "pdfs/table-curves-example.pdf")
+        with pdfplumber.open(path) as pdf:
+            page = pdf.pages[0]
+            assert len(page.curves)
+            tables = page.extract_tables()
+            assert len(tables) == 1
+            t = tables[0]
+            assert t[-2][-2] == "Uncommon"
+
+            assert len(page.extract_tables({"vertical_strategy": "lines_strict"})) == 0