marshal: don't escape quotes unnecessarily (#991)

Only 3 consecutive quotation marks need to be quoted. We choose to quote
all quotation marks in a sequence if there are 3 or more consecutive
present.

Fixes #990

---------

Co-authored-by: Thomas Pelletier <thomas@pelletier.dev>

marshaler_test.go

@@ -387,6 +387,54 @@ name = 'Alice'
 			expected: `A = """
 hello
 world"""
+`,
+		},
+		{
+			desc: "multi-line quotation",
+			v: struct {
+				A string `toml:",multiline"`
+			}{
+				A: "hello\n\"world\"",
+			},
+			expected: `A = """
+hello
+"world""""
+`,
+		},
+		{
+			desc: "multi-line triple quotation",
+			v: struct {
+				A string `toml:",multiline"`
+			}{
+				A: "hello\n\"\"\"world\"",
+			},
+			expected: `A = """
+hello
+\"\"\"world""""
+`,
+		},
+		{
+			desc: "multi-line triple quotation",
+			v: struct {
+				A string `toml:",multiline"`
+			}{
+				A: "hello\n\"world\"\"\"",
+			},
+			expected: `A = """
+hello
+"world\"\"\""""
+`,
+		},
+		{
+			desc: "multi-line sextuple quotation",
+			v: struct {
+				A string `toml:",multiline"`
+			}{
+				A: "hello\n\"\"\"\"\"\"world\"",
+			},
+			expected: `A = """
+hello
+\"\"\"\"\"\"world""""
 `,
 		},
 		{