Make editorconfig-checker exclude regex explicit

The `Exclude` array in the `.ecrc` configuration file contains the regular expressions of paths that should be excluded
from checking by the editorconfig-checker tool.

Previously, the file contained a filename that should always be ignored. Although it did work, it was not really correct
because the `.` in the filename is actually a regex wildcard, which is not what was intended. Although a false match was
unlikely, this also might mislead users adding project-specific exclusions to the file regarding the nature of the
exclude patterns. Changing to very explicit patterns avoids any chance of false matches and also makes it clear that
these are regexes.

Author: per1234

.ecrc

@@ -1,3 +1,3 @@
 {
-  "Exclude": ["LICENSE.txt"]
+  "Exclude": ["^LICENSE\\.txt$"]
 }