This commit is a squash of nearly 250 commits from the long-running
branch, `hash-lang`.

Major themes:

1. Change REPL I/O. We no longer use a TCP connection to do I/O for
each REPL. Instead use commands (input) and notifications (output).
Furthermore send various kinds of output as distinct notifications.

2. Support use of hash-lang colors, indent, navigation when editing
and in REPL.

Add racket-hash-lang-mode, an alternative to racket-mode for editing
source files, which uses coloring, indent and navigation supplied by a
lang.

Any number of racket-mode or racket-hash-lang-mode buffers may take
turns using the same racket-repl-mode. The last-run edit buffer's
settings are used in the REPL.

Needs Racket 6.12+ for interval-map-ref/bounds.

Use syntax-color/color-textoid when available (with new-enough
versions of Racket and/or syntax-color-lib) but not required.

3. racket-xp-mode: Do "semantic" highlighting of binding sites.
Intended for use by racket-hash-lang-mode to get more than just lexer
colors.

---

Fixes #482.
Fixes #619.
Fixes #642.
Fixes #663.
Fixes #667.
Fixes #671.
Fixes #672.
Fixes #673.

Closes #64.
Closes #633.

Closes PR #661.