Adjust heuristic

This commit moves the check for BOM at the start of the file and fixes a potential problem of compatibility with re2.
Note that `{3}?` in re2 is interpreted as matching the previous token exactly 3 times exactly while the Oniguruma engine interprets this as matching 3 or 0 times.

lib/linguist/heuristics.yml

@@ -89,7 +89,7 @@ disambiguations:
 - extensions: ['.bas']
   rules:
   - language: B4X
-    pattern: '\A(?:.*(?:\r?\n|\r)){0,9}^\W{3}?B4(?:J|A|R|i)=true'
+    pattern: '\A(?:\W{3})?(?:.*(?:\r?\n|\r)){0,9}^B4(?:J|A|R|i)=true'
   - language: FreeBasic
     pattern: '^[ \t]*#(?i)(?:define|endif|endmacro|ifn?def|include|lang|macro)(?:$|\s)'
   - language: BASIC