Autocompleters crash if the trigger has special meaning in a regex

[notnownikki]

Describe the bug

Autocompleters that use triggers that have a special meaning in regular expressions, crash the block they're used in when they're triggered.

78d1254 introduced the issue.

To Reproduce

Create an autocompleter with a regex-special character as the trigger. + will do.

Example code used to test:

const completer = {
        name: 'test',
        triggerPrefix: '+',
        options( input ) {
                return [ 'Johnny 5 needs input! ' + input ];
        },
        getOptionKeywords: function( input ) {
                return [ input ];
        },
        getOptionLabel: function( input ) {
                return '+' + input;
        },
        getOptionCompletion: function( input ) {
                return '+' + input;
        },
};

const appendCompleter = ( completers, blockName ) => {
        return blockName === 'core/paragraph' ?
                completers.concat( completer ) :
                completers;
}

// Adding the filter
wp.hooks.addFilter(
        'editor.Autocomplete.completers',
        'nikki/test/autocompletersbug',
        appendCompleter
);

Load the editor, and type + in a paragraph block.

cc @iseulde