This adds all of the stubs necessary to invoke the textDocument/hover LSP function, which is analogous to QuickInfo in Visual Studio. Currently only the C# kernel responds to this and it's responding with a hard-coded string. The work to produce the real content is forthcoming, but I needed the plumbing, first.

The LSP work is mostly split between two files. The first is lsp.js which handles the LSP-over-HTTP business. Not much to look at here.

The next part is specific to CodeMirror and how it's exposed through jupyter notebook. I've limited this PR to that case because it's simpler because any given web page has only one notebook open. That works by detecting the global JavaScript variables CodeMirror and Jupyter (not set in jupyter lab). Since CodeMirror doesn't natively understand a text hover operation, I've tied into CodeMirror cell creation and hook into mousemove and mouseleave to register timer-based callbacks that ultimately result in textDocument/hover being called if the mouse is in the same position for 500ms.

CodeMirror also doesn't have the ability to get the line/column of the character under the mouse cursor, so I've had to get creative. I do that by first iterating over the DOM elements that represent the lines of the editor. If one of those lines' bounding rectangles contains the mouse cursor, then I start to look for the column. Finding the column is much harder because the DOM elements don't represent anything useful, so I simply check each one in order, again checking the bounding rectangle. The end result is that the column that I detect is really the column of the first character of an arbitrary text span, but since CodeMirror is splitting the text on reasonable word boundaries, this will always yield the start position of what any programmer would call an identifier.

In the example below, the following markdown is returned as a hard-coded string from the C# LSP provider:

textDocument/hover at position (0, 9) with `markdown`

@colombod I re-worked the requirejs.config() call in HttpApiBootstrapper.cs so that the prefix dotnet-interactive points to http://localhost:<port>/resources. This makes it so that I can later call the following nicely:

require(['dotnet-interactive/dotnet-interactive'], ...); // returns http://localhost:<port>/resources/dotnet-interactive.js
require(['dotnet-interactive/lsp'], ...); // returns http://localhost:<port>/resources/lsp.js
require(['dotnet-interactive/code-mirror-lsp'], ...); // returns http://localhost:<port>/resources/code-mirror-lsp.js