Add functions to Variable related to variable substitution

[rcjsuen]

At the moment, Variable is defined like so...

export declare class Variable {
    getName(): string;
    getNameRange(): Range;
    getRange(): Range;
    isDefined(): boolean;
    isBuildVariable(): boolean;
    isEnvironmentVariable(): boolean;
}

It has no functions for:

• retrieving the substitution value for variables (the word replaced) for ${var:+replaced} and ${var:-replaced}
• determining whether a modifier has been defined, if it is a +, a -, or it is an invalid value (any character other than a + or a - cannot follow the : and is considered to be invalid, see Warn if variable substitution does not have a valid modifier dockerfile-utils#38)

[rcjsuen]

A new getModifier() function has been added to the Variable class in 448117a. I wrote the wrong issue number in the commit message so it didn't get linked here.