parse svelte constructs

[pngwn]

New svelte parser, echoes #324 but for Svelte. Replaces the solid but relatively unused (svelte-parse)

• Review svast to confirm AST interface #578
• Decide on strictness, i.e full html compliance or not

Constructs

• script #569
• style #570
• meta #571
• void_block #572
• branching_block #573
• each block #574
• element #575
• component #576
• comment #577
• Bare imports in markdown #433

Maybe solves/ need to write tests for:

• context module script detection seems janky #261
• "Expected valid tag name" when using special characters like < #76
• Cannot use custom (preprocessed) syntax in mdsvex files #116
• TypeError: Cannot read property 'parts' of undefined #118 (pretty sure this is just Cannot use custom (preprocessed) syntax in mdsvex files #116)
• HTML character entities throw "Expected valid tag name" error #296
• Cannot read property 'children' of undefined on build #377 (probably a remark version issue)
• Layouts Break MDsveX+TypeScript #485
• How do I group inline components in a paragraph? #430
• Keep HTML comments in render #498
• mdsvex does not correctly retain "chained" function calls #512
• Paragraph tag doesn't nest within an {#if} conditional #523
• Extend markdown support inside HTML / components #560
• named slots #263
• html entities, curly braces throw error #461
• Text expression syntax issues/collisions causing unexpected compilation errors #426