I found that if you make a single line edit in expressionEditor.mel then you will get a nice looking expression editor in maya.

Here is the change

[Python]
//~ string $path = \`scrollField -wordWrap false EEmultiText\`;
string $path = \`cmdScrollFieldExecuter -width 200 -height 100 -showLineNumbers true -commandCompletion true -sourceType mel EEmultiText\`;
[/Python]

Search for string $path = \`scrollField -wordWrap false EEmultiText\`; in expressionEditor.mel and replace that line with the new code. so now you have a expression editor with syntax highlight, line number, font changer with mouse wheel and more.

enjoy 🙂