Correctly handle CSV files with a single separator throughout

better auto-detection of CSV delimiter
- files with a tsv extension are automatically detected as tab delimited
- other files parsed as CSV go through the following steps:
  - if the first line contains at least 3 of the same separator, it uses that separator as a delimiter
  - if the first line contains only one supported separator character, it uses that separator as a delimiter
  - otherwise it falls back to treating all supported delimiters as the delimiter

 supported delimiters, in precedence order:
 - comma `,`
 - semi-colon `;`
 - tab `\t`
 - pipe `|`

tests/syntax-tests/source/CSV/simple.tsv

@@ -0,0 +1,3 @@
+foo	bar	baz|;,	test	hello world	tsv
+1,2	1,7	2,7	a b c	"hello	again"	tsv
+";|,"	;|,	baz	test	"hello world"	tsv