feat: maintain selection when switching from/to text mode

[josdejong]

To achieve this, we first have to implement two helper functions to convert from a text position to a JSONSelection and vice versa: function findTextLocation already exists, and this PR implements a new function findSelectionFromTextLocation.

TODO:

• work out findSelectionFromTextLocation in detail: it must return KeySelection, ValueSelection, AfterSelection, InsideSelection, and MultiSelection
• write unit tests
• in TextMode.svelte, extend the function applyExternalSelection with handling the case when externalSelection is a JSONSelection. Use findTextLocation there.
• In TreeMode and TableMode extend the function applyExternalSelection to handle the case when externalSelection is a JSONSelection. Use findSelectionFromTextLocation there.
• Test the performance in case of large documents. If needed, restrict maintaining the selection to small enough documents.
• Create and export utility functions to convert from JSONSelection to TextSelection