Improve signature help for functions with similarly named parameters

[dbanck]

Background

In #1077 we have introduced support for built-in functions. By default, function parameter highlighting is based on the parameter label. This leads to incorrect highlighting for functions that share a parameter name, such as format(format string, ...).

Expected Behavior

The parameter format is highlighted instead of the function name.

Actual Behavior

Steps to Reproduce

Autocomplete or type format(

Proposal

• Use [uinteger, uinteger] instead of string in ParameterInformation
• Introduce an updated ParameterInformation type
• Use the new ParameterInformation type in the whole request and response cycle
• Update ToSignatureHelp to calculate the index and offset of the parameter name in the function label