Implement command tokens and handlers in parser

[runarberg]

For now only font commands are commands, previously these were styles and implemented along with other prefixes. Now commands will become new binary prefix nodes in the AST which can apply special commands on the nodes below. For fonts this means to some text transforms or add style attributes. In the future we might implement color #40 and align #39 with command tokens.

Language Changes

• Now you can stack some font commands to combine them. For example, if you want bold italic A you can write bf it A. Not all font commands stack though e.g. fr bb will not give you double-struck fraktur.

Output Changes

• With the exception of mathvariant="normal" we no longer use the mathvariant attribute. Instead we apply the relevant text transform directly and the unicode characters in the output (see mathvariant is mostly depricated #70).
• Text gets some font commands applied via CSS (see Text literals should use CSS for certain font commands #78). Specifically this is bf, it, sf and tt on texts (e.g. bf tt "text"). The latter two change the font-family of the output. If you want control of sans-serif and monospace fonts in the output you can set the CSS custom properties --mathup-font-family-sans-serif and --mathup-font-family-monospace respectively.

Breaking Changes

• Font commands now only applies to the first item in a term, previously it was applied on the entire term (see Font commands should only apply to next token not row #77). bf A_(i j) would previously also boldface the indices (i j), but now it only boldfaces the matrix A.
• Font commands will no longer unwrap fences around the operant. For example if you write bf(A) previously the parenteses would be stripped, and you would only end up with A. Now the parenteses are left as is.

Resolves: #70
Resolves: #77
Resolves: #78