Relative paths, new path mapping setting, better diagnostics

[TibboddiT]

Motivation

• phpstan is sometimes executed in a php container, so most paths are more useful if they are relatives
• no indication of the origin on problems
• only the first char of the line is underlined when problem occurs

Changes

• Use relative paths where possible
• Add pathMappings settings used for resolving files path collected from phpstan output (comma-separated list of src:dest elements, with default: /srv/app:. to handle most docker use cases).
• Diagnostic range offsets (start at first non-whitechar and stops at last non-whitechar of line)
• Add source property on diagnostics, to get an indication of problem origin from UI