[PERF] Don't split and join lines when using TextFile

[TungstnBallon]

Problem: When chaining TextRangeSelector and TextLineDeleter the following happens:

// TextRangeSelector
let lines = splitLines(file.content);
let newLines = /* ... select range ... */;
return new TextFile(newLines.join());

// ...

// TextLineDeleter
let lines = splitLines(file.content);
let newLines = /* ... deleteLines ... */;
return new TextFile(newLines.join());

This might mean a lot of "useless" splits and joins, which becomes especially problematic for large files.

Solution: Introduce the new util function findLineBounds(). This function takes a sorted array of line indices, a regex to find line breaks and the text as parameters. It maps each line index to the start of the line and its length, stopping at the first non existent line index.

I didn't remove the now unused transformTextFileLines(), because deleting working, tested code seems unnecessary, but let me know it you think otherwise.