Cleanup function parameters and possible undefined

[ryanluker]

Based on error reporting version 2.0.0 has had a few crashes in these two functions, mostly undefined errors.

Check intersection being undefined before using reverse

vscode-coverage-gutters/src/renderer.ts

Lines 125 to 142 in 5eee5f7

	private findIntersect(base: string, comparee: string): string {
	const a = [...base].reverse();
	const b = [...comparee].reverse();
	// find the intersection and reverse it back into a string
	const intersection: string[] = [];
	let pos = 0;
	// stop when strings at pos are no longer are equal
	while (a[pos] === b[pos]) {
	// if we reached the end or there isnt a value for that pos
	// exit the while loop
	if (!a[pos] || !b[pos]) { break; }
	intersection.push(a[pos]);
	pos++;
	}
	const subInt = intersection.reverse().join("");
	return subInt;
	}

Check filename / name being undefined before manipulating

vscode-coverage-gutters/src/renderer.ts

Lines 144 to 152 in 5eee5f7

	private normalizeFileName(fileName: string): string {
	let name = fileName;
	// make file path relative and OS independent
	name = name.toLocaleLowerCase();
	// remove all file slashes
	name = name.replace(/\//g, "###");
	name = name.replace(/\\/g, "###");
	return name;
	}

Work:

• failing unit tests for above errors
• add proper ts types for the api (currently assume fileName: string when maybe it should be fileName: string | undefined)
• handle undefineds