Add textLength for precise character alignment

Use SVG textLength attribute with lengthAdjust='spacingAndGlyphs'
to enforce exact monospace character widths, fixing alignment of
box-drawing characters and other symbols.

Test coverage at 80%.

Bump version to 0.3.3

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "textual-webterm"
-version = "0.3.2"
+version = "0.3.3"
 description = "Serve terminal sessions over the web"
 authors = ["Will McGugan <will@textualize.io>"]
 license = "MIT"