Fix font rendering in browsers using WebGL/Canvas

Override xterm.js fontFamily via JavaScript since CSS cannot affect
canvas-rendered text. The terminal now uses the full monospace font
stack instead of falling back to Courier New.

Bumps version to 0.3.15

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "textual-webterm"
-version = "0.3.14"
+version = "0.3.15"
 description = "Serve terminal sessions over the web"
 authors = ["Will McGugan <will@textualize.io>"]
 license = "MIT"