Introduce tsserver for typescript code cells

[benjreinhart]

This PR replaces tsc with tsserver for reporting TypeScript diagnostics. Additionally, it does it on source change, rather than only at the time the code is executed, which makes for a more responsive editor experience.

This is the first step in a larger effort to have a full LSP running in srcbook with type definitions, autocomplete, reporting, etc.

Prerequisites before merging

• Move all user code files under src directory (Nest all srcbook code files under src directory #121)
• Make every typescript project have a tsconfig file inside the project directory (Generate tsconfig files for each typescript srcbook #122)
• Ensure the state after deleting is propagated (this pr)

TODOs

Lots of TODOs but some immediate follow-ups to call out:

• cell renaming needs to update tsserver state (Propagate updates to tsserver and client when a cell is updated #130, Use tsserver geterr command & events. reloadProjects for file renames #132, Updates to cell renaming behavior and UI #133)
• cell addition needs to update tsserver state (Rework cell creation flow #134)
• package.json adding/removing packages needs to get updated in tsserver (Update tsserver when deps are updated. Dont crash on missing tsserver #135)
• We're going to want to debounce source code changes. Right now, we run server logic on every key stroke. We'd want to do this regardless of tsserver, but given we're now doing more on the backend on every change, this is more of a reason to reduce the amount of server state updates. (Add client-side debounce for code cell editor state updates #129)
• We should stop treating TypeScript diagnostics as cell 'output' and instead treat it as first-class code cell metadata. The rules around diagnostics are not the same as the ones for stdout/stderr. This PR made that clear. (Treat diagnostics as separate entity from stdout/stderr #127)

[nichochar]

Looking good. Indeed using the output channel feels wrong, makes sense to migrate to its own channel as a follow up.