improve SourceLocation typing

[JLHwung]

Q	A
Fixed Issues?	Fixes #15775
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

In this PR we align the SourceLocation typing with the one in @babel/parser:

babel/packages/babel-parser/src/util/location.ts

Lines 8 to 31 in 386aaec

	export class Position {
	line: number;
	column: number;
	index: number;
	constructor(line: number, col: number, index: number) {
	this.line = line;
	this.column = col;
	this.index = index;
	}
	}
	export class SourceLocation {
	start: Position;
	end: Position;
	filename: string;
	identifierName: string | undefined | null;
	constructor(start: Position, end?: Position) {
	this.start = start;
	// (may start as null, but initialized later)
	this.end = end;
	}
	}

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/54849/