Exclude search results that are within the prompt

[meganrogge]

Now that we have shell integration, we should be able to provide beginSearchAfterTerm:string to ISearchOptions in xterm.js so that results don't include matches that are within the prompt.

For example, if prompt is prompt, findPrevious would only start searches at col 5 if the translateLineToString at the row being searched starts with prompt.

[Tyriar]

I don't think this is really needed? For any search of substance the prompt probably wouldn't be included and the user may also want to search the prompt (for the cwd for example).

[meganrogge]

Yeah I guess it's fine and is consistent with other terminals