switch repo to pnpm

[weswalla]

I am running into an error while working on #86. Firstly, declaration in the tsconfig is set to false which means no type declaration, which we want in the built client. When I change it to true, I then get the following error running npm run build:client:

src/widgets/dimension-widget-interface.ts:45:23 - error TS2742: The inferred type of 'AssessDimensionWidget' cannot be named without a reference to '.pnpm/@open-wc+dedupe-mixin@1.4.0/node_modules/@open-wc/dedupe-mixin'. This is likely not portable. A type annotation is necessary.

45 export abstract class AssessDimensionWidget extends ScopedElementsMixin(LitElement) implements IAssessDimensionWidget {
                         ~~~~~~~~~~~~~~~~~~~~~

src/widgets/dimension-widget-interface.ts:82:23 - error TS2742: The inferred type of 'DisplayDimensionWidget' cannot be named without a reference to '.pnpm/@open-wc+dedupe-mixin@1.4.0/node_modules/@open-wc/dedupe-mixin'. This is likely not portable. A type annotation is necessary.

82 export abstract class DisplayDimensionWidget extends ScopedElementsMixin(LitElement) implements IDisplayDimensionWidget {
                         ~~~~~~~~~~~~~~~~~~~~~~

src/widgets/sensemake-resource.ts:12:14 - error TS2742: The inferred type of 'SensemakeResource' cannot be named without a reference to '.pnpm/@open-wc+dedupe-mixin@1.4.0/node_modules/@open-wc/dedupe-mixin'. This is likely not portable. A type annotation is necessary.

12 export class SensemakeResource extends ScopedElementsMixin(LitElement) {
                ~~~~~~~~~~~~~~~~~

Found 3 errors.

This error seems to have been introduced in the switch to pnpm.