Use default export templates for HTML5

Changes Emscripten SDK version to match that used to compile the default export
templates used by Godot 3.5 (in this case 3.1.14 as can be seen here:
https://github.com/godotengine/build-containers/blob/3.5/Dockerfile.javascript).

This means we no longer need to compile custom export templates when
exporting HTML5 for Godot 3.5. Exports from other Godot versions may
requiring compiling the GDNative library with a different version of the
Emscripten SDK.

- Changes renderer from GLES3 -> GLES2 and reduces MSAA level to better
  support HTML5 export.
- Updates GitHub Action to export HTML5 and upload as a build artifact.
- Adds Cypress test to smoke test HTML5 export.

docs/CHANGELOG.md

@@ -6,6 +6,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 
 ## [Unreleased](https://github.com/lihop/godot-xterm/compare/v2.2.0...HEAD)
+### Changed
+- Custom export templates are no longer required when exporting to HTML5 from Godot v3.5.x.
 
 
 ## [v2.2.0](https://github.com/lihop/godot-xterm/compare/v2.1.1...v2.2.0) - 2022-08-26

docs/README.md

@@ -91,7 +91,8 @@ Are you using GodotXterm in your project and want it listed here? Please [open a
 ## Supported Godot Versions
 
 GodotXterm aims to support the current stable release of Godot, but may lag slightly.
-Currently, Godot versions 3.5.x and 3.4.x are supported.
+Currently, Godot versions 3.5.x and 3.4.x are supported, but custom export templates
+are required for HTML5 exports from versions < 3.5.
 Versions 3.3.x are no longer officially supported but *may* work if the GDNative libraries are
 compiled using an older version of godot-cpp.