fi-0188: Too many digits in Unicode escape sequence {:#fi-0188}

Unicode escape sequences in string literals cannot have more than 6 hex digits:

{% include “docs/reference/fidl/language/error-catalog/label/_bad.md” %}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/bad/fi-0188.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}

To fix the error, specify at most 6 hex digits. In this case, there are leading zeros that we can remove:

{% include “docs/reference/fidl/language/error-catalog/label/_good.md” %}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/good/fi-0188.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}

This restriction exists because all valid Unicode code points fit in 6 hex digits, so there is no reason to allow more digits than that.