The Lexer class for BBC Basic handles both the numbered lines, and
unnumbered lines, of the detokenised (text) format of BBC BASIC.
The tokeniser copes, in a naive manner, with the orignal versions,
and BASIC V. It does not handle other extensions at this time, nor
does it handle inline assembler. This should be sufficient for most
cases where code needs to be presented in a colourful manner.

Added lexer for VBScript

This introduces support for some missing features to the Handlebars lexer:
Partials and path segments. Partials mostly appeared to work before, but the
`>` in `{{> ... }}` would appear as a syntax error, as could other
components of the partial. This change introduces support for:

* Standard partials: `{{> partialName}}`
* Partials with parameters: `{{> partialName varname="value"}}`
* Ddynamic partials: `{{> (partialFunc)}}`
* Ddynamic partials with lookups: `{{> (lookup ../path "partialName")}}`
* Partial blocks: `{{> @partial-block}}`
* Inline partials: `{{#*inline}}..{{/inline}}`

It also introduces support for path segments, which can reference content in
the current context or in a parent context. For instance, `this.name`,
`this/name`, `./name`, `../name`, `this/name`, etc. These are all now tracked
as variables.