Implement undefined variable detection and prettify warning messages

[LucasSte]

This PR implements undefined variable detection for variables in function scope, using the current reaching definitions implementation. A few points need further improvement:

1. Reaching definitions does not work across functions, which prevents us from detecting new definitions on function calls, whose arguments are memory references.
2. Codegen Instruction Instr::AbiDecode needs refactoring to make reaching definitions work properly with try-catch statements.

In addition, the PR prettifies warnings and errors messages Solang prints to CLI. Here is an example of the new formatting:

examples/test.sol:24:13-18: error: unreachable statement
Line 24:
	a = 5;
	^^^^^

This work is part of the Linux Foundation mentorship for the Hyperledge Solang compiler. For more information, please refer to the project plan.

[seanyoung]

Looks great so far!