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/comma-delimited.csv

@@ -0,0 +1,3 @@
+foo,bar,baz,this|that,test,colors,cycle
+1.2,1.7,2.5,blah;cool,test,colors,cycle
+

tests/syntax-tests/source/CSV/decimals_comma_decimal_point_pipe_delimited.csv

@@ -0,0 +1,3 @@
+foo|bar|baz
+1,2|1,7|2,7
+1,5|8,5|-5,5

tests/syntax-tests/source/CSV/decimals_comma_decimal_point_semicolon_delimited.csv

@@ -0,0 +1,3 @@
+foo;bar;baz
+1,2;1,7;2,7
+1,5;8,5;-5,5

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