compiler/[msgs, suggest]: improve highlighter accuracy (#10496)

Previously the compiler would generate suggestions based on the symbol
identifier length, but that might not reflect the actual representation
of it within the actual source code.

This commit implements a simple source scanner for the suggest module to
address the problem outlined above.

Fixes nim-lang/nimsuggest#24

nimsuggest/tests/tsetter_highlight.nim

@@ -0,0 +1,10 @@
+proc `a=`(a, b: int) = discard
+10.a = 1000#[!]#
+
+discard """
+$nimsuggest --tester $file
+>highlight $1
+highlight;;skProc;;1;;6;;2
+highlight;;skType;;1;;16;;3
+highlight;;skProc;;2;;5;;1
+"""

nimsuggest/tests/tsi_highlight.nim

@@ -0,0 +1,11 @@
+proc a: int = 0
+e_c_h_o#[!]#
+
+discard """
+$nimsuggest --tester $file
+>highlight $1
+highlight;;skProc;;1;;5;;1
+highlight;;skType;;1;;8;;3
+highlight;;skResult;;1;;0;;0
+highlight;;skProc;;2;;0;;7
+"""